Leaving a car battery on a concrete floor won't do anything abnormal to affect the charge.
The belief stems from an outdated set of circumstances that no longer exist. Once upon a time, battery casings were made from more porous materials - tarred wood, rubber, etc. - which were susceptible to degrading after exposure to the moisture coming from unsealed concrete. As the casing degraded, battery acid could leak out and water could enter the battery, causing the battery capacity to drop.
With modern, plastic-encased batteries, the moisture-related degradation isn't really an issue on any meaningful time scales.
The reality is that a concrete floor is actually the best place to store a battery because the floor is normally cooler and batteries discharge at a slower rate when they are cool!