An earthen berm is a very cheap construction project. Classically those were achieved with hand labor worldwide, for millennia, eventually augmented with wheelbarrows starting in China, and oxcarts. They cost less today.
Nature providing one, two, three, or four walls (steepness optional) reduces construction cost, but the turbines and pumps are a substantial part of the total.
Opex is of course extremely low. Bonus points for solar farms floating on the reservoirs. Round-trip efficiency of pumped hydro is near the top of all alternatives, although efficiency is relatively unimportant when the power being stored was free.
Costs for many storage alternatives are falling fast, and it is unclear which choices will win. Adding pumps to existing dams has lately been cheapest, by far. Future people will choose according to local conditions without asking us.
The point is that whether a storage medium is practical in an area depends more on the landscape of choices than on the physical topography. Even Nebraska might have deep cavities that would serve in place of hills. But the extrema are extreme.
Like of course any water you push up a hill is gonna gain potential energy, but I think a reasonable reading of "works" in that post was "works efficiently enough that it's actually worth doing", not "is technically possible".