Now now... you can't expect trolls to be logically consistent, can you?
Besides, why he persists in calling Linux a UNIX clone, despite the fact that it isn't -- a fact that has been repeatedly pointed out to him -- is another story altogether.
It depends on the meaning. Pretty much anything that is POSIX compliant is sort-of a UNIX clone. In this aspect, even Windows is a UNIX clone. Anything that meets the UNIX specs isn't a clone, it's by definition real UNIX.
From what I know, Linux is mostly UNIX 03 compliant so certification of a version would be easy, but development moves so fast that it would be difficult to keep up with the certification.