Earlier quoted context omitted.
> It's a pity that she is working with FreeBSD, or it could have been of use to her. I suppose FreeBSD made more sense as a base considering MacOS is derived from BSD.
FreeBSD differs quite substantially from the XNU kernel used by MacOS because XNU is based on Mach, and it was forked (Edit: from 4.3BSD) in 1988 - before Linux even existed. The XNU kernel does not have a stable syscall ABI so perhaps it doesn't matter if the syscalls are different because the implementation of libSystem can convert as appropriate in userspace (see also: WINE).
Partially, XNU is Mach AND the FreeBSD-Kernel: