The "decentralized" nature of Git that GitHub interferes with is within an individual project. Instead of all the developers having their own copies of the repo and sharing things around between each other (i.e. the project is decentralized), there's a central GitHub repo for a project that is considered canonical and everyone interacts with that. This doesn't give GitHub any power to censor projects — Git itself is no more centralized than it ever was. You can use Git without GitHub and it works just as well as ever.