Plan 9 is Unix in the way that Unix is still Multics. So is Plan 9 still Multics?
Really, the superficial similarities should not overshadow the fundamental differences. QnX is also Unix by that definition, but in fact is a completely different system under the hood.
Compared to the others I've mentioned? Sure, and that would most likely mean that most systems are closer to Unix than e.g. SmallTalk. Probably even including Windows, unless one would have a very favorable view of COM.
Really, the superficial similarities should not overshadow the fundamental differences. QnX is also Unix by that definition, but in fact is a completely different system under the hood.