They did, but (IIRC, and this is an old memory) to make GNU/Linux pass certification you have to hack it up quite a bit; lots of GNU tools have POSIX modes that significantly change how they behave - things like du defaulting to 1024-byte units by default but using 512-byte units if you set POSIXLY_CORRECT because parts of the standard are, um, "interesting". But yes, neat to know that it can be done.

AFAIR, Red Hat once had a version of their system certified, too. But it's not as useful as one might think - the certification applies to a specific distro and release only, so it needs to be renewed with every new release.