Why do the two different "ground effect"s used by aircraft and automobiles do opposite things in almost the same conditions?
venturi effect occurs until the venturi is so extreme that the flow is "choked" and the venturi effect ceases to operate and you just have an air cushion? Is that the answer?


No. Neither answer to your linked question talks about venturi effect at all!

In the aircraft case the wing is still far enough above the ground there there is no significant venturi effect at all. The pressure field around the wing is still similar as in free flight, but because the air can't move down behind the wing, the effective angle of attack is decreased, which reduces the induced drag. It does not even increase the lift all that much, reduction of drag is the most significant effect.

In the car case the bottom of the car is much closer to the ground, but the venturi effect is still not the major part. The underside of the car acts as an inverted wing, which creates upward lift. The ground again prevents the, in this case, upwash, this time by preventing other air to fill in and this again reduces the induced drag. The point is again not creating more force, the point is creating the force with less drag, so the car can go faster with the same engine (most racing classes are limited by engine power or cylinder volume).

Also note that venturi effect would be always downward, so if it was true that venturi effect increases until it becomes “choked”, it would pull the aircraft (which has the wings higher, so definitely no choking) down and possibly (depending whether it would be choked under the car) push the car up, the opposite what is observed.