to just be playful banter
Jesus that's very generous. It's "playful banter" in the way 4chan uses the term, or Donald Trump does, maybe. It's toxic behaviour, plain and simple. The only thing up for debate is whether they're doing it out of ignorance or, as I suspect, because they deliberately want to drive out the kinder voices.
bants with mates at the pub
Are usually, I dunno, funny? They're also contextual. You don't rib a mate when he's looking for sincere feedback, you read the room. In their case, they're reading the room and seeing a prime opportunity to prey on the vulnerable.
Why do you think that including one's pronouns in a username or bio is transphobic
It's not. The way they use this incredibly inclusionary tool to make fun of the concept, however, is. Saying their pronouns are "comrade" and "use name" makes a mockery of the concept. It reminds me of that old transphobic "attack helicopter" joke.
Hexbear is a very large and old instance; up untill a couple of months ago they were running a version of Lemmy that did not support federation
Is it theoretically possible that this is the case? Sure. But again you're being very generous to a group of people who have demonstrated themselves not to deserve that generosity. Especially when we're talking about people who care enough about the issue to comment on it in this thread.