The poster I replied to stated (I remember) that some use of language is made legitimate by some group laying out some guidelines. That some pretty random group («not philologists») lays out guidelines is maybe an "affiliation pass" for their group, but not universally valid.

You need grounds, good grounds. Check those guidelines...

Some people: "let's use, of already-established pronouns, the one that doesn't make any assumptions about people" You: "no, fuck you, other people's interpretation of what I say is strictly their problem, I'm gonna stick with 'he' no matter what other people feel or request, because I can"

The only ideology I can see here is the absolutist sort of "nobody can even suggest to me that I change a thing" or "I can't possibly cause offense if I don't mean to, so I don't need to think about word usage."

I don’t know what Michael’s preferred pronouns are, but isn’t the original poster in this chain assuming it’s they/them and aren’t they more likely, statistically speaking, to be he/him?

Why does it somehow not count as misgendering when you they/them someone that prefers he/him or she/her?

Because one appears to make an assumption about gender, whether or not you intended it to be taken as a generic, while the other objectively is generic.

As far as options go, they/them minimises assumptions. Yes, some people might still take offence, but given that most of the people who take offence at that takes offence because they oppose inclusiveness of others, I'm perfectly fine with not respecting their choice in the matter.

You have some evidence of this or just the unshakable certainty of the self righteous?

If you are an exception and have a good reason for taking offence that doesn't involve excluding others, do tell.