You know about the kernel but not how to use your compose key‽

The fact that the kernel is mirrored on GitHub doesn't mean they use it in any sense like the way most projects use it. The real point is that the way Linux is developed has no relation to that GitHub mirror, so it's an example of open source development outside of GitHub.

I don't think anyone is claiming that GitHub can't host a mirror of a big git repo. That's not what parent is claiming is "too big".

The problem is that the tooling for reviewing changes on GitHub doesn't scale and also doesn't match the model that Linux uses (deputies). Hell, I would argue large projects like Kubernetes and Rust are hitting these same issues.

Also, that is just a mirror. The real hosting is on https://git.kernel.org/.