Does threads not use the same usernames as Instagram? Like if I sign up, using my instagram account it wouldn't be the same?
It does, yes, I think you can just log in with existing Insta without problems. But not all of those wanting an escape from Twitter at this stage have Instagram accounts, or at least not in the same ID.
btw, it seems across-platform squatting is becoming increasingly worsening problem. horrible people just take IDs they've seen elsewhere, just-like-that.