His dream of twitter is a town hall where you can come say whatever you want.
This guy works with him, clearly isn't professional, so he fired him. He is free to work anywhere in the world, just not at a place he owns.
He didn't censor him, he didn't delete him, he just chose that this guy isn't a team player at his company.
Wtf does that have to do anything about free speech? If you say **** I don't like, and I PAY YOU, then I'm firing your ass, you can go say the same stuff at someone else's place.