I think "faster discharge" rates is being noted for their ability to dump large amounts of current very quickly. This is useful for large surges of power. It's not referring to some sort of parasitic leakage.
Consider what happens when the flywheel dumps all of its energy within a fraction of a second - a not-unlikely event in the case of almost any mechanical failure. Do I have to spell it out?
This phrase is the key to understanding why flywheels are seldom used. Read up on what happens if the wheel jams, cracks, or otherwise malfunctions.