There is also craned power generation BTW.. Concrete blocks are stacked with a crane.. They are then dropped to the ground and the potential energy harvested by the cranes motor
Which falls under the category of component sizing. But assuming you have however much mass at a given height, a denser fluid won't let you extract the same amount of energy from a lower height, which is what the company is claiming.
Volume matters for the sizing of components, it doesn't matter for the amount of energy you can extract from a height differential.