To answer onesheet's question, it's because Mel is pretty amazing at ferreting things out and in finding great buys. I have many, many posters that are very rare, and that you could say the same thing about. Authentic does not necessarily mean readily available. But, you can believe what you want to believe. It leaves more for us!
To answer skyjacker's question, you're entitled to believe what you want to believe. Also I have posters that were not supposed to have been printed according to one source, but were printed because I have them in hand.
As collectors, we reach the point of being able to make independente assessments, and to not rely exclusively on what other people have to say. I have posters that merely because of the way they smell I can tell you they are or are not authentic. Christ, I can pretty much tell you what country they are from by the way they smell!
My strong advice to each of you, and to anyone else who's reading, is to take to the bank what Mel has to say, because he knows what he's talking about. Of course, you may ignore him too, and that's ok. But, you'd be missing out.