Those stories are rare. For every claim about a rust free car that has no rust there are 99 examples to show disproof.Agreed, that is the most best way to make sure it doesn't rot underneath.
I have heard stories of people though, who've driven their cars for decades and they still look brand new from undercoating. Mind you, they wash the outside of the vehicle constantly and spray it themselves to know where exactly the hidden spots are!
All these things like oil spray, Krown and Rustcheck only work on where they are sprayed. There are hundreds of places on a car it can not reach. Where is does causes a huge mess which is better than rust and frozen bolts. Even winter alone is brutal on a car. How do you wash a car during the sub zero temps? Dry it and okay, wait for a warm day when the road are wet then drive it home. Rinsing underneath does not remove all the salt. There are so many cavities a car wash will not reach.
My advice is for non car people to oil spray their car to help it last longer. Any car that is trying to be preserved needs to be not exposed to salt at all.