Wow! Linus is using a Macbook Air!
It's almost certainly running linux though.
https://lkml.org/lkml/2004/2/12/13
Many open source OS projects have core developers on non-x86/amd64 platforms so as to find errors, x86isms, and other code quality issues reveled by fundamental differences in the underlying architectures.
And before that, he used an iBook running OS X to write his autobiography on MS Word (he said so in an interview).
The bit that really caught my attention was this: "(...) Torvalds, who briefly moved Linux kernel development to GitHub last September following a security breach at its old home."
I must have been away when that happened.