I'd argue that 'massage' is the right word in this case.
I've this technique a lot in japanese cooking -- you cut up the vegetables, sprinkling with salt as you go (so there's layers of salt in between layers of vegetables), then you really get in there and basically massage (knead?) the pile of vegetables with the salt, so that the salt not only is spread ...
Yes, this is mostly same as rub in with salt, but with a bit more intensity and physical contact with the food. While rubbing can be taken as applying one coating, massage warrants ensuring that the salt is mixed well with the food.
It's done so that the sprinkled salt is spread uniformly.