Bitcoin Core is not only in GitHub. Every person that has ever cloned the repo has a full copy of the source code and the history of changes. That's how git works.
It's also available from bitcoincore.org, and from there you can see that it's also available as a torrent.
And that's just the most common implementation of Bitcoin, but there are other implementations, like Bitcoin Knots, btcsuite, libbitcoin, and others. Note that these are not forks of Bitcoin, they are Bitcoin because they follow the same rules, they are just a different code base.
Yes, having the current code is decentralized. But the distribution method is a broadcast from a centralized source.
Also, almost everybody is using the same implementation instead of just the same protocol.
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