There it clarifies that it is "mostly" FreeBSD. The Mach being used also differs significantly from CMU Mach and what you might find by looking for Mach3/4 source dumps.
One of the biggest challenges in "converting" NeXTSTEP (MacOS's predecessor) to OS X was both updating software to newer versions and eliminating expensive licenses from AT&T and Adobe.
NeXTSTEP was a "capital U" UNIX with AT&T proprietary code, based on 4.3/4.4BSD (encumbered). Every copy needed a UNIX license and royalties to AT&T.
NeXT was based on Mach2, which had 4.3BSD deeply integrated into the kernel source tree. Device drivers were both native BSD ones along with a "DriverKit" interface that used Mach messages to write userland device drivers.
CMU Mach v3 and v4 cut out all the BSD code and put it into a userland "UX Server", a model incompatible with NeXT. So instead, Apple took the OSF/1 Mach kernel, derived from Mach 2.5, and replaced the BSD subsystem code with 4.4BSD-lite, gradually updating its subsystems with FreeBSD ones.
So TLDR, Darwin/XNU has both a BSD userland and essentially a FreeBSD kernel. When you make a "UNIX-y" syscall from C in MacOS, you're "talking" to a "FreeBSD kernel".
[1] https://github.com/apple/darwin-xnu