> Being fully compliant with the POSIX standard isn't quite enough; you have to pass an official conformance test suite and be certified by the Open Group to have passed it, in order to call yourself "Unix", rather than just being Unix-like.

Technically, Unix certification and POSIX certification are separate, you can certify as either.

Unix certification: https://www.opengroup.org/openbrand/register/

POSIX certification: https://posix.opengroup.org/register.html