Due to incompressibility of liquids, pressurizing a liquid is very cheap energy-wise - orders of magnitude cheaper than pressurizing a gas. The issue is that pressurized liquid also requires correspondingly strong and expensive vessels and pipes.

Well, if you say the energy stored is the 100MWh from the headline figure, and say you can arrange release every joule of all at once by flashing high-pressure water to steam at 1 atm that's about 0.1kT.

So quite a bang - allegedly this is 200lb, so about the same: https://www.youtube.com/watch?v=ZDgvar7ON54