I think part of it might also be from the Tulpa theory and the idea that Slendy actually was created on Something Awful. Marble Hornets was the first Slender series and, going by this theory, MH would've probably had a very large hand in making Slendy real, due again to its popularity.
Though there is one thing with the blogs claiming MH isn't real that I think would somewhat contradict one thing in the blogs. It's been sort of established that looking into Slendy too much or spending too much time on Slendy related things usually causes him to show up. Even if MH was fake in the blog universe, after two years I think Troy and Joseph would be much overdue for an actual visit by our tall, dark and slender friend.
But anyway, I'm just rambling.