yeah, the scalability of this seems like a pretty big challenge

annoyingly, they talk about the amount of water they pumped only in terms of energy (35MWh) and not in terms of water volume.

I think they do that because, if you estimate the water volume...it's pretty unimpressive.

going off the numbers for Bath County Pumped Storage Station, the largest in the US, and until 2021 the largest in the world:

total storage capacity of 24,000 MWh - meaning that this power station built in the late 70s / early 80s has almost 700 times the storage capacity of this 35MWh demo

between their upper reservoir and lower reservoir, their water capacity is 78.4 million cubic meters. so as a crude estimate, Quidnet's demo project used ~115,000 cubic meters.

Olympic swimming pool contains 2.500 cubic meters. so, again with the caveat that this is a rough estimate because Quidnet didn't publish the actual numbers...this demo they're bragging about involved 45 Olympic swimming pools worth of water.

Obviously, assuming you have that hill.
Slightly harder to do in places like the Netherlands for example, where the tallest hill is 322 metres, and the second tallest that isn't part of that same mountain range near the Belgium border is just 110.

And in the US, Florida, Delaware, Louisiana, Mississippi, Indiana, and Illinois are actually flatter than the Netherlands - sure, the highest point in Indiana (Hoosier Hill) is 383 metres from the sea level, but the lowest point in the entire state is 98 metres above.