RR only considered this to be P2 because he wrote the script long before the real P2 even came out. He didn't like P2, yes, I understand that. So he wanted to do his own, direct sequel to Predator, ignoring P2, AVP, and AVPR. The thing it, he didn't contradict anything in those movies, and the director Nimrod Antal even stated that this was a sequel to Predator 2. Currently, all the Alien, Predator, and AVP films can fit together in one canon, regardless of whether or not the filmmakers only like certain films. Nothing contradicts anything else yet (Unless you are like me and fail to see any logic in eggbarfing, but that is a discussion for another thread). Now lets wait and see what Prometheus does to that...
Also, about your Predator 6 comment, AVP and Predator take place in the same universe, but are not a part of the same film series. So Predator 6 makes no sense, whereas Predator 4 does. Despite having canonical relations to and shared setting with the AVP films, they are not the same series.