While that is technically true, the fact remains that if you want an area of terrain to be non-bombable, all you have to do is make it too thick for the number of bombers the player can afford to use, whether that means changing the terrain, lowering the number of bombers available, or increasing the save requirement.
(There is one exceptional case, where terrain being bombable is a problem because a bomber could dent it rather than breaking all the way through it -- for instance, making a dent where a climber can stop climbing and build out. However, if that would create a backroute, in most cases steel or a flamethrower etc. would be sufficient to prevent it.)
NL already has a glut of features, and if we are going to add yet more, we should restrict it to ones that really bring something new to the table and open the way to genuinely new puzzle ideas.