The roots of this topic was that someone told me Haiku is not a UNIX-like (at least internally) based on the code supposedly not following the Unix system design for the most part and documentation. They explained a bunch of other things, but I can’t find the chat where it happened. They also said that Haiku is as much of a *nix-like as z/OS (with its UNIX-certified POSIX compatibility layer), OpenVMS, and Windows NT are; POSIX compatible but still not *nix-like beyond that (such as the internal structures).
EDIT: If I can find the chat again, I’ll let you know.