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.