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.