Gravity isn't the storage mechanism; the ballast bag is. Gravity is the mechanism used to unleash the stored energy, the same way that the electro-chemical voltage difference between the two components of the battery is the mechanism that unleashes the stored energy in the battery. If anything, it's a ballast-bag-powered light.
I'd say you have the most accurate description of when they say "Gravity Powered". While lifting this thing, the energy gets stored and gravity is just a means by which the stored energy is released.
