You may have been kidding, but you've hit the nail on the head haha. All cars are shaped like wings....the airplane kind that generate lift. Any object that has more surface area on one side vs. the other will create either lift or downforce, the side with the less surface area sees the pressure. If you flattened the surface area of the top of a car it would be much much longer than the bottom surface area which means pressure builds under the car and pushes upward creating lift. An airplane wing is designed the same way where the top half has a longer surface area than the bottom. An automotive wing is basically flipped upside down so the pressure is a downward force.
That being said, sticking the laminar air flow to the back of the car for a longer period of time actually creates more lift. So there's a trade off at some point. In one of those links they use the example of a modern VW bug which is a perfect example of aero fail despite the laminar flow of air over the rear sticking all the way to the license plate basically. As far as the rear diffuser goes, the vertical fins are basically VGs. Going back to the length of the surface area thing, rear diffusers slope upward and the fins help air stick to the surface creating a longer surface area on the bottom of the car so it better equals the top of the car basically producing negative lift, not so much downforce.
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