Actually, Mach and BSD have bupkis in common. It simply so happens that OS X uses a reasonable amount of BSD user-land code in OS X to make compatibility easier. This probably makes porting FreeBSD to clang easier indirectly--the user-land tools and libraries from the BSDs are written in the same style as the kernel, and so supporting them likely makes supporting the kernel easier that much easier--but saying Mach is similar to BSD is like saying Windows NT is similar to DOS simply because it can run DOS programs.
You're technically correct in regards to Mach, but in the context of Mac OS X a very large percentage of the kernel is composed of BSD-derived code.
Unfortunately, none of this is particularly relevant to clang, as the kernel design (in respect to Mach or BSD) really has little bearing on its compiliability.
[Edit]
I'm very surprised that this was downvoted to 0. The discussion was in context of Mac OS X, and the technical veracity is easily confirmed:
