I have read this before but have never read the NRA publication, you are refering to. The theory is that metal parts would expand with the use of hot water. Water would fill cracks and small voids and then cooled would seal them in. As and example, the threads in a breech plug. I no longer give it much though as I stopped using water years ago. I'm thinking that this is why I never recommend anyone trying to remove this plug on a side-lock. .....
Will have to go back in some of my old books and see what I can find. ...
Be Safe !!!