What I mean is, on hearing that I thought of a conveyor belt whose top surface was moving at a speed -x with respect to the air, and a plane on top of it moving at a speed x with respect to the top of the conveyor belt, i.e. the plane was stationary with respect to the air. But on reading the Snopes link what was actually meant was that the conveyor belt was moving at speed -x and the plane's engines were working as hard as needed to move at speed x on stationary ground with no wind.