As an artist myself I find that claim almost insulting. A forger might have great technique, but in as far as he copies art rather than creates it from scratch himself, he's just acting as a sophisticated xerox machine or camera. I really don't see what lifts that sort of mechanical act even to the status of art, much less a greater art than that of the artist who created the original.
Also, to suggest that the marketing by art dealers or art's reception by consumers has any bearing on the "greatness" of the art they're buying and selling is less than convincing. Sure, there is more and less popular art, and art that rich people (many of them mere speculators) are willing to pay a lot for (often simply because they calculate they'll make a profit when they sell). But what on earth does that have to do with how great the art is?
If Van Gogh's art had remained as obscure and unappreciated as on the day he died (having only sold a single painting in his life, and that to his brother), would his art have been any less great?
It seems ridiculous to me to claim that the vagaries of the art market determine how great an work of art is.
To me art dealers and other middle-men are parasites and signs of inefficiency and friction in connecting consumers directly to producers (artists), and a necessary evil because so many artists hate business and want to focus on their art. That critics and dealers play a role in determining how popular art gets is symptomatic of the art world's pathology, not something to be praised.