Github is just a coordination tool, which can be switched at any time. Git itself is decentralized already.

I agree that admin control of the github repo could be seen as "centralization" by some, but you have to consider that you don't have to download and run every new release that is put on that website.

The consensus rules will work just fine for you if you ran an older client. Hence the importance of avoiding changes that break consensus rules, AKA hard forks.