The binary portability is, in practice, not the most difficult feature, as long as the CPU arch is the same. It is also kind of a hack and IMO a nice-to-have feature but not as vital as true portability.
POSIX was not limited to the Unix world, the goal was for it to be implemented by all OS vendors, and it was partially done.
Doesn't matter. POSIX is "write once, compile everywhere," while this is "compile once, run everywhere." It could be that POSIX is easier to write than for Cosmo bins, shifting the balance between them! I see them as just different endpoints of development.
POSIX was not limited to the Unix world, the goal was for it to be implemented by all OS vendors, and it was partially done.