This is one of those "not all men" "not all white people" things. It's common practice to speak in generalization about groups that have a general consensus of chauvinism (whether demographics, organization members, or whatever). One thing that is good about this is that it encourages the relative outliers to not identify with the group (e.g. you can't 'opt out' of being white, but you can stop personally self-identifying with the social formation of whiteness). When a .ee or .ml user disparages their instance, they are not disparaging themselves or literally every single person in it (despite the denotative meaning of the words they use) but are pointing to a broad and explicit consensus around certain views and traits. Incidentally, there are groups that you can opt out of, whether it's being a member of a bourgeois-democratic party ("not all Liberals") or something more trivial, like being a user on a neoliberal instance. I think it's better to stay on one's instance and try to change it, but it's worth noting for the sake of completeness, and there are some instances that are not worth trying to change except to kill, like .world

We can language police all day long, but this line is old-hat and there's little reason to believe any wording would be received as "correct" and "properly inoffensive" except for silence. A "possible ally" who is alienated by something this trivial wasn't ready to be an ally anyway.