Definitely not decentralized, also don't tweet anything NSFW because Github will ban your account without hesitation, removing all issues you created, even for projects you don't own. Source: happened to me because I had an "NSFW" Github page that displayed randomly generated Imgur pictures.
That's why you need to split "list of all people" (necessarily centralized for discovery) and actual storage (decentralized). Everyone can have their own repo anywhere as long as it's fetchable by git, and the creator of the tool can maintain a list of all repos branches
