Not true....if you boil rust you change form of the oxide to one that is much more stable....rust blueing for example uses this process. It comes down to not all guns can end up having perfect bores because the walls are not always thick enough. Some guns also are not worth the cost of putting through the back bore/honing process. We all like pit free, shines like a mirror bores, unfortunatly not all guns can be put in that condition and maintain safe wall thickness or be cost effective for said gun. I wouldnt keep myself up at night worrying about the deepening of pits over time. Like I said earlier....simply putting the gun through the boiling process converts the oxide, both on the outside and the inside of those pits.
Steve