Which generates more energy, dropping 1000 tons of water 100 meters, or dropping 1000 tons of steel 100 meters?

Storage tanks are limited by volume, not mass.

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.

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

But as they stated it translates into higher energy storage capacity (per unit volume). This translates into smaller footprints as volumes go down, but that’s it. Now, mercury is almost 14 times more dense than water so that’s nothing to sneeze at.