It's a kernel module implementation of much of OSF Mach verbatim copied and shimmed from MkLinux, not just IPC. See sys/compat/mach in the NextBSD source tree.
Yeah I looked; it's the IPC from Mach with some shimmed bits to do VM-y things for mapped message passing.
Mach was mostly VM and IPC - so of course it looks like it's a lot of Mach code. But there's no separate mach kernel running - it's just the IPC instances and they only exist as FD bits. It's not a complete kernel and FreeBSD isn't running "on" mach.
I never made such a claim that FreeBSD is running on Mach. I said it integrated a Mach kernel interface as a module running in kernel space.
They also ported tasks (as processes), threads (kthreads) and some clock/timer stuff, as well. No memory objects because there's no external pager they can be backed from.