The other day, I stumbled across this post. After reading it, I thought to myself, if you have to ask where your enneagram type leaves off and your Myers-Briggs type begins, then you probably don't know enough about the two systems to type yourself on either one. Unfortunately, a lot of the confusion is the by-product of the "grazing" approach to information where people avoid immersing themselves in source material (something which, with my five wing, makes me seriously cringe, but I'm trying to make my peace with it) preferring to quickly sort out small, relevant parcels.
The danger inherent in typing oneself based on such little knowledge is indicated by the way the person identifies with a lot of the anxiety and validation issues described by Palmer, yet when they get to the authority issues, choose to dismiss them as neither relevant to the post nor to themselves. Still, if you are familiar with the enneagram, you know how vital the authority issues are to type Six. An INFJ Six that has a lot of fear and doubt but can't identify authority issues probably isn't a Six at all. Rather, they are simply experiencing the high degree of anxiety typical of INFJ. (INFJs, in addition to reporting the greatest degree of marital dissatisfaction, also have anxiety in the greatest number of areas in their life. I'll get into the statistical correlations between the MBTI and validated Enneagram tests later in this post.)
Maitri states in The Enneagram of Passions and Virtues:"Many people who study the enneagram erroneously decide that they are Sixes because of the degree of fear, anxiety, distrust, and doubt that they live with, but each of the nine types has its own kind of fear and all nine can be seen as different responses to survival anxiety." She goes on to explain the different ways the types experience fear:
If we are afraid of creating conflict by making ourselves or our needs too obvious, it is likely that we are a Nine. If we are afraid that there is something fundamentally wrong with us or that who we are is not enough or good enough, it is likely that we are a One. If we are afraid of rejection, of being needy, and of not being loved, it is likely we are a Two. If we are afraid of failure, it is likely we are a Three. If we are afraid of being abandoned, of our sadness, and of feeling lost, it is likely we are a Four. If we are afraid of entanglements and of losing what we have, it is likely we are a Five. If we are afraid of grunt work and of being exposed as a charlatan, it is likely we are a Seven. If we are afraid of being weak and not being in charge or on top of things, it is likely we are an Eight. If, on the other hand, we are simply afraid of everything and everyone to one degree or another, if fear itself in a nameless, faceless way is the driving force of our psyche, then it is likely that we are a Six.
This nameless, faceless thing has a lot of implications. Indeed, not all Sixes easily recognize that fear is driving them. One reason is that, when fear is driving the psyche, one tries to find a way to silence it. In fact, all types could be said to do what they do in order to silence whatever their basic fear happens to be. For Six, this is where authority issues come in. It can be any kind of authority issue: it can be the authority of the group, it can be policies, religious beliefs, social norms, a specific ideology or actual authority figures of some kind. There is often a hypersensitivity to authority issues, particularly from those that may not be trusted, and this will vary based on the Six. This same fear, though, also drives the person to seek out community (broadly construed) and unite with others in a way that will confer a sense of safety. Helen Palmer (who the OP was quoting, albeit indirectly) addresses this by describing activists uniting in defense of a cause. Maitri is less generous, describing the group orientation of Six as a "fearful banding together" though I think the banding together, while often in a tightly knit social group, can also be manifest in finding a sense of safety through larger-than-life structures. Regardless, the group or structure stands in contrast to some other, outside force that is perceived as threatening.
I think part of the reason why you're finding a lot of INFJs quickly typing themselves as Sixes is because they hear about not just anxiety, but skepticism and doubt, which make Sixes seem more intellectual than some of the other enneagram types. Actually, different enneagram types may be intellectual, but in different ways. Talking about enneagram Four In Character and Neurosis, Naranjo states:"We are in the presence of an 'emotional type' just as in the case of ennea-type II, only here with a greater admixture of intellectual interests and introversion." For type Six, though, he describes an intellectualism and a commitment to reason that leaves little room for ambiguity. He states:
Unlike ennea-type VII who uses intellect as strategy, type VI is likely to worship intellect through fanatical allegiance to reason and reason alone--as in scientism. In his need for answers in order to solve his problems, type VI is more than any other a questioner and thus a potential philosopher. Not only does he use the intellect for problem solving, he resorts to problem seeking in order to feel safe.
Here, we see type Six as, say, the scientist or scholar that is intimately tied to what is verifiable and supported by evidence. It also may be the analytic philosopher that wants things reduced to a logical proof. With an orientation towards problem seeking and solving, the kind of intellectualism of Six could also be described as the techie who works in an area (such as information security) where they not only have to solve problems but anticipate them. Naranjo's description is admittedly very NT-ish, and I've seen my share of NF Sixes that get into continental philosophy, particularly as it connects with feminism and race theory. Still, the take-away from all of this is that type Six's intellectualism serves as an orientation towards certainty and detecting flaws, problems and inconsistencies. By contrast, type Four's intellectualism is more exclusively based on a personal connection to intellectual subject matter, to the point of being self-referential in one's pursuits. All NFs seek meaning in their intellectual pursuits and all NFs are concerned with the human condition, but with type Six, problem-solving and certainty play a larger role,whereas with type Four, the connection serves the self-image in some way. Think Descartes' Meditations versus Goethe's The Sorrows of Young Werther.
But what types, when we're talking about scientifically validated tests, are associated with INFJ or type Six? Enneagram author, Jerome Wagner, did a study in which he correlated MBTI results with his scientifically validated WEPSS test. His sample sizes appear to be adequately large (type Two, for example, contained 200 participants). In his study, type Six was correlated with ISFJ, not INFJ. For type Four, the greatest correlation was with INFP, not INFJ. INFJ didn't directly correlate with any of the nine types, so we can assume that INFJ is not so close to any one enneagram type as to be thought as indistinguishable. In fact, if you're INFJ and type Six, then you more likely than not find at least some of the Six descriptors to be ill-suited to you, since a lot of Six descriptions emphasize the introverted sensing aspect of it.
Based on that data, I'd propose that many people of a given type will vary by one letter from the dominant Myers-Briggs type for their enneagram type. Some people that either don't neatly fit into the the enneagram or aren't a good match to the Myers-Briggs may be off by two letters and a very small percentage may be off by as many as three.* So, based on that, we could expect type Four to most likely be INFP, followed by INFJ, ISFP or ENFP. For type Six, we could expect to most commonly be ISFJ, followed by ISFP, INFJ, ESFJ and ISTJ. I don't mean this as a way to commit people to having to be certain types but rather to help them gain a better understanding of how the enneagram and Myers-Briggs interact. Are you an INFJ and don't know where INFJ ends and type Six begins? Simple. INFJ's dominant function is introverted intuition but most people who test as type Six have a dominant function of introverted sensing. Don't know the difference between type Four and INFJ? Simple. Most people that test as type Four introvert their feeling whereas you, as an INFJ, extravert your feeling.
*In my estimation, I'm also considering the rate of agreement that people typically have with their MBTI results, where most agree with at least three of the four letters, so some of the people that test as ISFJ and Six might verify as INFJ and some that test as INFP and Four might verify as INFJ.