> A very large portion of that ABI is already implemented due to both systems being POSIX.

POSIX provides an API, not an ABI, and that API kind of ends at libc. Being compatible with Linux at an ABI level means being able to provide the same syscalls in the same way as Linux does. Not all Linux syscalls map cleanly to POSIX APIs, and in general xnu has lots of different concepts that make it somewhat cumbersome to adapt to what the Linux kernel does. The example of this is Microsoft with WSL1; they gave up not because Windows was too shoddy but rather because people want ALL of the kernel, which is a moving target anyway. it's a waste of time not to simply run it in the first place, virtualization is cheap and you get the real thing, with no quirks