XNU began as a microkernel, but has become more monolithic as it became clear that certain functionality really needed to be in the kernel for performance reasons. I now includes significant stuff from BSD.
Note that MacOS is unix! Despite the slogan "X is not Unix", MacOS has UNIX certification. It is compliant with the SUS (Single Unix Specification) and thus holds UNIX 03 certification from the "Open Group" which holds the UNIX trademark. This began in 2007 with MacOS 10.5 (Leopard).
Amusingly, linux does not hold formal Unix certification. Something to bear in mind is that this would involve significant licensing fees. Who would pay them. And who would care?
Adventures in Computing / tom@mmto.org