Right, salt is ionic.

Salt absorbs water, and then it does something called caking. Which means it glues into large clumps of crystal when wet then dried, instead of reverting to tiny cubes. The idea of the rice has nothing to do with moisture but to prevent the clumps after the salt dries with a physical barrier. If you shake it in place after it dries, the rice also mechanically break the lumps. It's an old restaurant trick because they use commercial table salt, which is specifically made for and restaurants has no anticaking agents (usually some form of silicate) and no iodine, so it's taste is not metallic. That's how all salt used to be. But modern household table salt does include it, and as result doesn't cake and it remains granular no matter the moist.