That's missing a good chunk of the interesting stuff related to how users interact with the os though. (app and framework source code)

Yeah, apps and frameworks aren't part of the kernel. But APFS is.

It's loaded into the kernel, but it's a kext (kernel extension) rather than being part of the xnu source tree itself. Only a subset of kexts are open source. (By comparison, HFS used to be in the xnu tree, but it was moved into a kext as well a few years back; that one is open source.)

Kind of talking out of my ass, but Darwin can act as a mirokernel, so APFS could be implemented as a user mode service (no idea if it's the case, though).

Darwin's XNU is not a microkernel, in spite of there being a microkernel version of XNU. While Darwin does support FUSE, performance would almost certainly be inadequate, given the constant transitions between user and kernel space.

I thought it was a mixture of OSFMK and BSD, which is why I said it can act like a microkernel. Don't know more than what wikipedia says, though.

Yes, it is, but the Mach layer and BSD layer were fused into a monolithic kernel before NeXT Step turned into OS X. In OS X, HFS (and presumably APFS) are in the BSD layer.