I think part of the trouble is that queer has different meanings for different people. You have done a good job describing the umbrella definition, which is very important. When I joined Queer Nation 1990, we begin working to reclaim the word in the sense of referring to all members of gender and sexual minorities.
And in many respects it’s still used like that today. But there is another increasingly popular, more specific definition. When some people call themselves queer, they mean that they do not identify strongly with other labels. For example, somebody who calls themselves genderqueer, might not feel like they fall within the gender binary.
Someone else might feel like homosexual and bisexual just aren’t good enough to describe how they experience sexual attraction. So they stick with queer instead.
Then there are definitions of the word that predate even 1990, definitions that arose in the academic world of queer and gender theory.
So it all gets complicated!
As far as the fellow you ran into on Quora, my initial guess would be that he is an older gay man who grew up experiencing queer only as an insult and just isn’t on board with the reclamation.
I know a lot of people like that. They are firmly opposed to the word queer under any circumstances.
Personally, I think people should adopt the identities and descriptors for themselves that they like. But I get a little bit annoyed when some folks try to tell others which identities they aren’t allowed to use.