Github is a completely optional thing though. If it were to die tomorrow, everyone still has the code locally, and code sharing is still very much possible.
That is also not what is meant by decentralization in the blockchain sense. A decentralized use of git would be a graph with no central hub: each developer syncing their repo with one or more other developers directly, not with a central "source-of-truth" repo. In reality if Github disappeared everyone would just find a new host to centralize on. Furthermore Github creates lock-in by hosting data that actually would not survive any migration attempts: issue tracking and pull request discussions.
The main reason to mention github at all is that it gives me stats. There's probably a bunch more repositories out there just from people doing git clone, but I can't count them.
Also, while github helps it's not a critical part of the whole thing. If it disappeared it wouldn't be a critical problem. Everybody would still have the source, and could figure a way to reconnect again.