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/.