https://opensource.apple.com/source/xnu/xnu-4570.71.2/bsd/ke...

https://opensource.apple.com/source/xnu/xnu-4570.71.2/bsd/ne...

https://opensource.apple.com/source/xnu/xnu-4570.71.2/bsd/ne...

https://opensource.apple.com/source/xnu/xnu-4570.71.2/bsd/ne...

You will find RCS tags for OpenBSD/NetBSD/FreeBSD littered throughout the kernel source tree, some files even retaining original SCCS ids from 1980's CSRG days.

    $ grep -R "\$OpenBSD:" | wc -l
    24
    $ grep -R "\$NetBSD:" | wc -l
    138
    $ grep -R "\$FreeBSD:" | wc -l
    162
    $ grep -R "\$KAME:" | wc -l
    73
    $ grep -R "WIDE Project" | wc -l
    87
    $ grep -R "The Regents" | wc -l
    427
    $ grep -R "Berkeley" | wc -l
    919

This isn't "userland (POSIX) API layer + the odd device compatibility layer", far from it.

Don't presume to know who you're talking smack to, when it's clear you have absolutely no idea what you're talking about.