Because Github does not want to be complicit in Chinese government censorship, and if they blocked what that government obviously wants them to block, then they would be.
I think you are missing the guy's point: what is the difference between Russian censorship and Chinese censorship? Github made a deal with Russia - why not also China?
Didn't know about that, that's make the difference. Usually in totalitarian countries everything is well documented as everybody want to put responsibility on someone else.