The moving vehicle analogy is often presented to explain how Earth’s rotation does not seem to affect an object’s motion. Similarly, a ball released inside a moving vehicle will not fall rearward because it retains its inertia.
Flat-Earthers offer the moving open-air vehicle as if it is the “correct ” analogy. In reality, with their analogy, the air moves relative to the vehicle, which exerts an additional force that does not exist in the fact being explained. Their analogy fails to represent the fact being explained.
Earth’s atmosphere follows the rotating motion of the rest of the Earth. When a ball is released, the ball already possesses the same speed as Earth’s atmosphere and its surface. When it is released, the ball will preserve the motion. Because there is no force other than gravity affecting it, the ball will fall straight downward.
The same thing occurs inside a vehicle moving with a constant direction and speed. The air is stationary relative to the vehicle and the object inside it. It will not affect the motion of a falling object horizontally. From the reference frame of the vehicle, the object will fall straight downward.
On the other hand, with an open-air vehicle, the air moves relative to the vehicle, exerting drag to all objects on the vehicle. It will alter the horizontal motion of the object. Because this additional force does not exist in the fact being explained, then their analogy is faulty.