I think what they're trying to say is that water doesn't drain until the soil is saturated; until that point pretty well all the water is in the soil (ie above the gravel). Just as if you put a damp sponge on top of a layer of gravel, it would stay damp, and wouldn't drain all its water down into the gravel. The gravel wouldn't get appreciably wet until the sponge was sopping wet and the water started falling out of it. So all you're doing by adding a layer of gravel is making the pot smaller.