If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform).
If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.
On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.