BeOS on its final commercial version certainly did not allow to compile UNIX applications, beyond the common surface that is part of ISO C and ISO C++ standard library.
maybe in early BeOS versions but, BeOS R5 especially with the BONE updates had a fairly decent POSIX compatibility for the time. If you do "ls /" you can see immediatly BeOS has some BSD reminiscence, but certainly it isn't a UNIX OS as in itself.