It says exactly one right thing: you can put a reservoir on a hilltop, and the "head" is the height of the hill, not the depth of the reservoir.

You do not need a mountain. You do not need a high valley. It is cheap to build an earthen dike around the top of a hill, leveling off the hilltop peak for material.

The hill does not need to be steep; a shallow slope just means a longer penstock, which costs more.

A 300-meter hill is high enough for practical use. In most cases only a few hours' storage is plenty; you only need enough to make firing up a gas generator an occasional event.