Ich is not a disease I would make a conclusion like that based on. Fish are immune to it until stressed. If they were stressed by something (slightly bad temp during water change for example) it might stress them just enough to allow that one spot of ich to develop but the fish recovered on its own well enough to fight it off without assistance. If the salt was killing the ich it would not have started growing in the first place. If the salt wasn't too much to keep the ich from growing it wasn't enough to kill it.