It's just such a miniscule amount of power.
Do you want to know why you don't see power dams below a certain size and height? Because gravitational potential energy is not very much. And I say this as someone who gets more excited by the Niagara falls power station than by the actual waterfall.
I also question their math claiming 200w per sq meter.
https://www.insidescience.org/news/how-much-power-can-we-get...