I think this title is misleading. Apple has open XNU for more than 10 years. And AFAIK this kernel does not include ARM compiling option, as the very first paragraph in the README:
> XNU runs on I386, X86_64
> XNU runs on I386, X86_64
There's also some ARM specific code: https://github.com/apple/darwin-xnu/tree/master/pexpert/arm
The Makefile also has ARM/ARM64 options.
This guy works on Foundation: https://twitter.com/Catfish_Man/status/913983607777116160