I don't think XNU is a microkernel in any real sense. Certainly not in the sense that the Minix kernel is or was.
The filesystem, device drivers, IPC, network stack, etc is all in kernel space. It's true that it's built around the Mach microkernel, but like most systems that built around it, Mach is just a core layer in the kernel onion, rather than a microkernel in and of itself with the other major components implemented as user-space servers.
A low-level IPC mechanism needs to exist in the microkernel. Higher-level IPC mechanisms that applications will actually care about and use certainly don't need to; see, for example, pipes in Minix.
The filesystem, device drivers, IPC, network stack, etc is all in kernel space. It's true that it's built around the Mach microkernel, but like most systems that built around it, Mach is just a core layer in the kernel onion, rather than a microkernel in and of itself with the other major components implemented as user-space servers.