Leftnut is speaking of "adverse yaw" and that's when the down moving aileron causes enough lift (and therefore drag) to pull the plane 'round the other way from the turn input.

Nothing to do with the plane just yawing and not turning.

Aileron differential is done to counteract adverse yaw, by setting the ailerons so the upgoing one moves up more than the downgoing one moves down.

That's not the problem here though.