No one ever said only soaking would cause shell rot!I have also been under the impression, that if you soak in clean water and then afterwards you dry your tortoise/the shell dries under the heat lamp shell rot wouldn't be a problem. It is hard to believe that done right soaking would be the only cause of shell rot...
Yes, if you dry the shell plastron mainly before putting it back in its enclosure then it's not wet bottom sitting on damp substrate. Most don't dry their tortoise first, cuz it's better for most tortoises to not dry them. Those prone to rot probably should at least dry the plastron but I haven't ever seen that mentioned as something that should be done. It would also likely depend on the substrate. Is it wet on top or just on the bottom layers? Each owner needs to be sure they are following what's best for their species not just what is the general rule.