Lift comes primarily from more air molecules hitting the wing below it than above it per unit time.
I don't think so. Imagine a flat bottomed airfoil with a positive camber on top that is positioned so the bottom is directly parallel to the relative airflow. There are no air molecules "hitting" the underside of the wing yet the wing is still able to produce a significant amount of lift in this position. There would be skin friction on the underside of the wing which would cause the molecules closest to the wing to slow down to a very slow speed and as you move further away from the bottom of the wing you'd notice the speed would increase progressively until you reached the free stream air speed.
When you're talking about a piece of plywood or a barn door flying, it would have to be at a VERY low angle of attack to fly by the same principles as an airfoil normally does. This is because the critical angle of attack would be very low due to the lack of a smooth and progressive path for the air to follow as it flows around the top. The change in velocity (direction in this case) would be so high that the surface tension would not be high enough to be able to impart enough force to keep the airflow attached to the upper surface of the plywood. It would break away at the critical angle of attack and it's now stalled. To make the plywood fly beyond this point, then you would be relying a lot on the molecules hitting the underside of the plywood and imparting a force based on Newton's laws that would cause the plywood to "fly". Important distinction is that this is not how airfoils normally fly!