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).
By the time Apple got to them it was a few years later, and so they decided to updated that part of the kernel, and also brought in FreeBSD's userland.
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).