Except... Haiku is POSIX? The POSIX compatibility isn't just some compatibility layer, it goes all the way into the kernel. We use the fork-based process model, POSIX filemodes, etc.
What does "strictly POSIX" mean, and how does Haiku not meet it?
If there are POSIX APIs we don't implement, it's simply because we haven't gotten around to implementing it / nothing needed it so we didn't bother, not because we are explicitly avoiding said APIs.