I mean, you could even do git peer-to-peer via ssh. Each person sets up sshd and a bare git repository, add it to their remote and now they can push to each others "ssh" repositories and then fetch from it when wanted.
Sure, or with patches broadcasted on radio, scribbled on napkins etc.
I don't think you intended to counter my argument but in case anyone is interested in elaboration: my point about centralized/decentralize is that using github doesn't necessarily centralize your work. It's only central in the sense that any other remote repository is: if you can't access it you can't. If github dies, copies of the repository may still exist on a million other computers and any of those can be used as a basis for further collaboration by a wide variety of means. From a peer to peer perspective, github is just a peer like any other.
