Einstein's statement that you don't feel the downward acceleration when free-falling refers to the experience of weightlessness during free fall in a gravitational field. In a vacuum, where there is no air resistance, objects of different masses fall at the same rate, as famously demonstrated by Galileo's experiments. This is known as the equivalence principle, which is a fundamental concept in general relativity.
However, the sensation of dropping in the stomach that skydivers experience is not due to the downward acceleration itself, but rather the effects of air resistance. When skydivers jump from an aircraft, they initially experience a sensation of falling because their bodies encounter air resistance, which opposes their motion. As they accelerate, the air resistance increases until it reaches a point where it balances the force of gravity acting on the skydiver. This is when they reach terminal velocity, where the net force on the skydiver becomes zero, and they continue to fall at a constant speed.
The sensation in the stomach that skydivers feel is due to the forces exerted on their body by the air resistance. The rapid change in velocity and the interaction of air with the body's surface can create sensations such as a "drop" in the stomach.
On the Moon, where there is no atmosphere or air resistance, free falling would not cause any sensation of dropping in the stomach. Without air resistance, objects would fall at the same rate regardless of their mass, just as in a vacuum on Earth. Therefore, a free fall on the Moon would be akin to a true state of weightlessness, without any sensation of falling or stomach drop.