I think technically speaking you do not need upward force to create spin. If you hit with more forward force on the top of the ball vs the bottom of the ball then the net effect would be that the ball rotates with the leading edge being the side that was hit with more force.
Also I guess you can consider the timing. The top hits first, starts to rotate and at the same time might roll down a little (or squash down) on the racquet. By the time the back of the ball impacts the string bed the top might be already on it's way out (rebound and rotation since the bottom might still be in compression stage). That would mean that to top is already rotating the ball and leaving the racquet so the bottom of the ball contribution would be less right?