As another commenter noted: XNU = Mach + FreeBSD.
What you are referring to is what NeXTSTEP was, Mach + BSD4:
* https://en.wikipedia.org/wiki/NeXTSTEP#Unix
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.
As another commenter noted: XNU = Mach + FreeBSD.
What you are referring to is what NeXTSTEP was, Mach + BSD4:
* https://en.wikipedia.org/wiki/NeXTSTEP#Unix
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.