It would make sense if carbon had any protective qualities... It is like saying that you don't need to oil and clean your firearms if you covered it in soil because the layer of soil will protect the rifle. Don't feel bad, I see the same fellows that say this advising people not to wash their salt covered vehicles during the Winter because the dirt on the vehicle will protect it. Too bad that theory is easily proved wrong by looking at their car.
I don't think carbon has any protective qualities, but it's more like a layer of soil "might" protect it from a layer of salt a little bit more than nothing at all being in the way. And like i said, it's a big might, and it's more about my #2, which means if i want to shoot 20 rounds just to let someone new try it out i can use non corrosive and not have to do a full cleaning immediatly. If i'm going to be letting loose hundreds of rounds in an afternoon and it will be getting a good cleaning anyway, it'll be corrosive because it's cheaper, and if the initial carbon buildup helps in any little sort of way it's a bonus.