Two separate ideas being debated here.

1. Ufer grounding as an appropriate or legal method of grounding your system.

2. Concrete floors considered to be at ground potential.

For the latter, I have always been told that you must consider a concrete floor to be at ground, and therefore refrain from touching any hot wires.

I have not attempted to measure that myself and don't know the source of the claim. But I've seen it in a number of electrical safety classes, so if it is an urban legend it is a well distributed one.