heat treated either means to boil the driftwood or bake it in an oven. it's to further insure it's aquarium safe. im pretty sure it will still release tannins but if you soak for a week, there should be less tannins.
if your buying driftwood and it says heat treated i would guess it means it was heated up. kind of like boiling it or something. prob put in a oven or something.
Grape wood usually just rots when fully submerged. It starts to grow white fungus on it and stinks up the tank. These woods are usually meant for reptile homes not full water aquariums.