i see this as a simple as an object on rollers. like the kind factories use to move stuff around in assembly lines. yes the smog dyno rollers are not level so it would require more force but the car would move off the rollers.

because in order for the roller to keep the car in it's place (not roll off) the wheel have to be the one creating the force (ie rotating under the engines power). however when an outside force acts upon the car as a whole, the car rolls off the rollers. just like in my example above the rollers on an assembly line turn but the box on the rollers move off the rollers they were on before you pushed the box.

this is a similar if not the same idea/principle as the airplane on a conveyor belt one.