If I refuse Nazis access to my house and other people chose to visit my house knowing I won't let Nazis in, I'm not censoring Nazis as I have no influence on their freedom of speech - they remain entirely free to speak elsewhere, to people who are willing to listen. I have no power to stop them from speaking, just to offer those who don't want to listen somewhere where they won't have to.
Nobody is forced to join a Mastodon instance that censors Nazis - there are many that don't, or you can run your own.
I never used either Twitter or Mastodon and don't quite understand what people use these platforms for. But I hear folks saying they are there because other people they know are there.
How would it work on Mastodon if you want to communicate with two people that happen to have each others instances blocked? Is it like having to use both Twitter and BlueSky or does it solve the problem more elegantly?