"vast" is a pretty non specific word so I can maybe see why you would think that? I feel like it's pretty obvious from context I'm not talking about an inland sea here. The quoted 5000 "acre-feet" in that project is a considerable amount of water for a man-made structure!

Either way, the details page[1] supports all of my above points. It even comments on the page how rare it is to find a suitable site like the one they've chosen.

[1]https://www.whitepinepumpedstorage.com/project-details