Yep, it looks like they have a kernel module for handling the Mach subset of syscalls.

> Emulating XNU system calls directly in Linux would have a few benefits, but isn't really workable. Unlike BSD kernels, Linux has no support for foreign system call emulation and having such an extensive and intrusive patchset merged into Linux would be too difficult. Requiring Darling's users to patch their kernels is out of question as well.

I wonder what such a patch set might look like.