That doesn't say that everything related to Windows is developed on the same repo, just the kernel and several core components.

How did I miss this! Can you imagine someone claiming Windows is going to be in git as few as 10 years ago? This world never ceases to amaze me. What happened to SourceSafe?

SourceSafe was never used for anything internal major. They had their own custom source control system, based on Perforce, I believe, and then on TFS and then they switched to git.

SourceSafe was something inflicted on SMBs :-D

Interesting, didn't realize that. I haven't worked there but knew some people who did, and they were very proud of dogfooding - I guess at least the OS and IDEs, though maybe not the VCS.