Isn't the kernel's repository just mirrored on GitHub, though? That's a bit different than hosting Python primarily on github.

Correct, but no part of the Linux kernel development is done on GitHub. It's nothing more than a republishing of the patch-based authoritative repository hosed somewhere else. The issue is that if you want GH to be your authoritative repository you're locked into GH's way of doing things -- which isn't necessarily bad, but a valid complaint is that you have to be a user of GH to contribute to projects hosted there.