Git is decentralized as is. It stores all the history locally and sends a copy to GitHub AND/OR any other repo you would like. GitHub is just a part of decentralized git ecosystem. It simplifies communication. But I agree - it is good to use more than one repository. And in case of Bitcoin code it is crucial to use several different private repos owned by different people.
Not decentralized in the way in which we are speaking. Sure with Git you can have a local copy and others who are working on it will work on their own local copy that is stored thus providing some form of decentralization. Still it is not fully decentralized in a way that it would be censorship resistant, IE theoretically say some government body got wind of you working on some code which they do not want you to work on and raided you and your co-workers you would have nothing protecting you once each local copy has been deleted, in this case there would not be many copies as you are only distributing it with you co-workers there is no trustless party who has copies of your code.
What we are looking at is a solution to prevent a malicious actor from being able to delete the copy locally from only say 5 computers and then your work being gone forever.
The way Git is set up is not meant to have your copy live forever no matter what, it is set up to only have your copy live as long as needed. Huge differences.