Its decentralized nature isn’t due to censorship, or a lack of it, but its structure. Of course there’s going to be some degree of censorship because instances all have individual rules which, if you break, you’ll be penalized for which can take the form of removal of content.
The only way to have total free speech is create your own instance which is a total free for all but then you’ll attract the worst sort of people and your instance will end up defederated by instance owners who don’t want content from literal Nazis federated to their instances.