Sure but it's just the kernel, and that's just a mirror. Linus does not use Github to manage kernel development. In fact he's been vitriolic in the past about how Github does pull requests.
I wonder how much traffic the Github Linux repo gets. Seems to me that people who want to use Linux, will go get a distro instead. And people who want to develop the kernel, will follow the kernel development process (which doesn't rely on Github).
For a period of time when Kernel.org was breached, GitHub was the repository for Linux. [1] I remember reading a review of GitHub by him shortly after. He did not like how Pull Requests or patches worked on GitHub. I'd link it but I'm having trouble finding it.
