It doesn't matter if an instance wants their instance to be "the" instance for a community, they don't get to decide that for the whole Fediverse. It's up to the users.
It's also one of the concerns I have about user impersonation
Usernames should be interpreted in the same context as email address. I am FaceDeer@kbin.social, the "@kbin.social" part is as much a part of my username as the "FaceDeer" part.
The UI should probably be displaying it more prominently than it is, that's a bug that Kbin should resolve at some point.