Technically, it's immune to non-magical slashing/piercing/bludgeoning weapon attacks. It's not immune to falling damage for example (according to JeremyCrawford at least; I would rule differently), and the rot grub isn't using a weapon attack.

But none of that matters, because now you've got me thinking about the awesomeness of a Tarrasque which is killed by the players... and then the corpse splits open to reveal that it's infested with millions of rot grubs.