I don’t think that that’s hair-splitting (if a group Y is, by definition made up of ‘people with characteristic X’, assertions of the form ‘Y people are so X’ is so tautologically true as to be beyond question, unless your motives/affect/context for bringing that fact up are so overtly crass that doing so is basically the equivalent of just making rude noises while somebody else is trying to talk.
However, I suspect that this view is a bit naive about how things like religions and political parties actually work, as social institutions. Even in situations where Orthodoxy, rather than Orthopraxy, is the alleged gold-standard (eg. Protestant Christianity, strongly self-selected dissident groups or college Randroid clubs), it isn’t uncommon to find people who will affirm whatever ‘correct’ positions they can remember, if prompted; but embrace them with such minimal fervor that they are totally untroubled by holding or acting on a variety of other positions that are blatantly contradictory to their alleged positions. In situations where Orthopraxy is actually acceptable (rather than just inevitable), all bets are off. As much as I find people who exist in this state of blatant contradiction and/or compartmentalization rather galling, their existence is something empiricism has forced me to accept.
Outside of rather contrived examples that read like exerpts from a textbook on how deductive logic is both irrefutable and nearly futile as a way of gaining information about the world, even membership in allegedly ‘like-minded’ groups tends to encourage; but not to require, any particular characteristics (unless party leadership is ruthless and efficient at weeding the rank and file). ‘Cafeteria Catholics’ who pick and choose from the buffet without the slightest regard for theological consistency or statements from HQ, nominal adherents of various political parties who loath at least half the candidates thereof, or are overtly wrong about what the candidates claim to stand for, much less what they do, that sort of thing.
TL;DR, my tendency is to be very suspicious indeed of the existence of a bright line between ‘merely incidental’ and ‘like-minded’ groups: it could easily be (and probably sometimes is) that you can draw stronger statistical inferences from mere demographic data than you can from affinity-group membership in the sloppier groups.
That said, one major difference is that bringing up how statistically anomalous an individual is, relative to their group, is a common ‘stereotyping’ tactic with regard to groups that are seen as involuntary ( in US usage, say, calling a black person perceived as atypically focused on educational achievement an ‘oreo’ (black on the outside, white on the inside, ‘bananna’ serves the same function for asian/pacific colorations) or accusing them of ‘acting white’; in class contexts, anything in the ‘you think you’re too good for your roots/family/us?’, that sort of thing). That sort of thing is pure stereotyping, and can be quite effective.
By contrast, with groups that are ostensibly defined by affinity, and can be changed, it would not obviously be stereotyping to ask “Umm, so you don’t agree with X, Y, or Z of the party platform, and your main interest is really what the W party does. Why do you call yourself a member of the B party?” Similarly (especially if socially salient issues are on the line), asking somebody why they continue to stand behind a group that endorses some invidious policy, despite not agreeing with it themselves, and reminding them of the fact that they are voting with their feet, so they really ought to have good reasons, isn’t ‘stereotyping’ in any useful sense (though it can be obnoxious if overdone). If, say, a liberal Catholic of your aquaintance, despite not believing that catholicism is the only route to salvation, or that homos and uppity women are icky, remains on the membership rolls, and possibly even kicks some cash into the offerings plate from time to time, asking them "What are you doing supporting somebody who puts their power and influence directly against what you believe? Is that ethical? You can get almost the same service down the street with the Episcopalians, and the UUs are always taking members, what’s the deal?)
That aspect of affinity groups (that they can be changed) seems like a much bigger deal. Allegations to the contrary, people fall into ‘affinity groups’ for no better reason than “well, that’s where mom and dad took me growing up” or “everybody did X in $HOMETOWN$”, so judgements based on group membership are just statistical best guesses, same as any other; but critiques of group membership with respect to stated beliefs and/or behavior with such voluntary associations seem to be non-stereotyping, while the same critiques with respect to involuntary groups are a very potent form.
(Incidentally, if you want a total bloody mess, that lives right on the boundary between ‘involuntary’ and ‘affinity’ groups, just look for a fight between ‘deaf culture’ advocates who think that Cochlear implants are the slow-burning genocide of their equally valid way of being and Cochlear implant enthusiasts. Since the best time for the operation is on the relatively young, you get the extra salience of people making decisions in loco parentis for minor children to go along with it.)
