It seems to me pumping out water to near vacuum while under crushing outside water pressure would be a significant power hog

Well, yeah. That’s the point. It’s a battery. Whatever energy you put in to pump the water out, you get some percentage (probably in the 50-70% range) of it back when you let the water back in. The point of these is to store energy from renewables whenever they are providing more power than the grid demands - otherwise the power would be wasted.

Edit: The paper claims 72% efficiency which is pretty good if I understand things correctly