If you look at the contents of that repo, you will see that the majority of the code there is headers to allow compilation of very specific parts (e.g the net a

If you look at the contents of that repo, you will see that the majority of the code there is headers to allow compilation of very specific parts (e.g the net and inet directories), not the whole codebase. Certainly, there is no kernel there and most calls redirect back into xnu. Most functionality is Mach-based, this is just for a user-facing API.