There is a grain of truth in what you have beed told. Assuming very specific situation - 1.5" rod, that have to be eaten uniformly to left just a few milimeters rod that can be bent by hand, you need around a pound of concentrated acid per inch of the rod. This is simple stoichiometry.
In fact you don't need acid for that, it is enough that steel rods are too close to the surface of concrete and they can be reached by water that always slightly penetrates concrete. In the presence of water iron will corrode producing oxides that have higher volume than original rod. That means pressure, and pressure will burst the concrete (albeit slowly) making cracks through which more water and oxygen can reach the iron. Process speeds up then, and corrosion proceeds along the rod into the voulme.