But do they have to submit a request if they tell the audience “fuck it, this is now a sub about X, we’ll remove everything that’s not about X”?

No that just isn’t allowed. I’m not joking, the admins have removed entire mod teams and installed new ones because the mod teams decided to change the topic of the sub.

Hexbear is older than most of the fediverse, and didn’t have federation enabled for years. It’s a very self-sufficient community.