> Offriver closed loop pumped hydro just needs two football size fields hundreds of meter apart in elevation
A football field sized reservoir isn't going to hold much water unless it's deep. Realistic pumped hydro sites are much larger than 500 square meters. And those two reservoirs can't be too far apart, because excess lateral distance reduces output and increases the energy required to refill it.
It also needs to be close to a source of water in order to fill the reservoir and refill any loss due to evaporation.
It also needs to be close to a major highway otherwise the costs of construction will be immense because of the long distance that materials and equipment must be shipped. This is particularly problematic because most pumped hydro potential is in mountainous terrain where transportation is expensive.
And lastly, this site has to be close to places that have energy demand, otherwise you'll need to build miles and miles of transmission lines. This is also a problem because the places that have pumped hydro potential also have hydroelectric potential, and usually already generate electricity from dams. Norway and Brazil have huge pumped hydro potential, but already generate most electricity from dams.
The conditions for a viable pumped hydro sites are much more narrow than "two football field sized reservoir a few hundred meters apart in elevation".
The paper you cited just ran an algorithm over a terrain height map. It did not attempt to account for accessibility. Good luck developing the immense pumped hydro potential in Tibet. Pumped hydro would be great if we could teleport concrete, excavators, and turbo machinery into some of the most remote parts of the world. But unfortunately this is not in our capabilities.