It would seem that flywheels can store a lot of energy, but due to the form of the energy storage (kinetic) it’s difficult to discharge the current quickly, so to speak. Especially compared to something like a capacitor which is designed for rapidly discharging a literal current, as opposed to the analogous metaphorical motive current in a flywheel.
The flywheels GP refers to were homopolar generators [1]. They created an impressive current pulse, and extracted almost all the kinetic energy in less than one revolution.
