Edit: my below comment was actually wrong. They actually do use git.

Thanks for sharing. What I find most interesting is that Linus is still using the same email-based software development methods for the kernel while the rest of the software engineering world has evolved to use his other invention, git, for that. I’m kind of second-hand embarrassed for those geniuses who have yet to adopt proper version control for (what I’d argue is) the most important project in the computing world.

Here’s a far more nuanced explanation from Spore’s reply to this comment :

Git and Email are not mutually exclusive. In order to collaborate with git, you need and only need a way to send your commits to others. Commits can be formatted as plain-text files and sent through emails. That is how git has been used by its author from literally the first release of it.