A strong BeOS selling point was that it was POSIX compliant. I guess that is what you mean by a thin veneer. But we had to write all that code; it wasn’t like we were running Linux or some SCO or other flavor of Unix. It was a written and rewritten and rewritten multiple times.
I am greatly enjoying the juxtaposition of a comment asserting that obviously BeOS was so far from POSIX that Haiku had to do loads of work to get there and your comment that BeOS was POSIX compliant.