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.
Haiku is all new source, so they had to figure out how to get the binaries to run. I am not the POSIX expert, but I think it was just an API guarantee not ABI.
There were several companies that had the BeOS source and that source can be found in the wild. It is speculated that yellowTAB ZETA was based on unlicensed BeOS source code.