They don’t need personal information. If the owner of a server doesn’t like what you post , they can ban the account. They run the whole machine so if they really wanted to they could block your IP from even connecting in the first place!

Decentralization doesn’t mean you’re free to post whatever you want. It’s still someone else’s computer, and they make the rules. Federation doesn’t change that. Sure it’s harder to suppress information, but a few admins could easily coordinate against you if they so wanted.