Um… of course it will still be able to take off. The velocity of the air over the wings (plane speed relative to a static position roughly) is the ONLY thing that matters. How fast the conveyor or its wheels move is completely irrellevent.
However, they need to specify that the conveyer is going backwards at TWICE the speed of the plane. (plane speed -plane speed = dead stall, plane couldnt even move, you need to substract plane speed AGAIN to make it the opposite of plane speed)
As I said, if the plane if moving forward on the conveyer at takeoff speed relative to the ground, the speed of the conveyor and wheels (-2x plane speed) is meaningless… though you better not be twitchy on the controls!