I think that's bogus, 20% of rated voltage is can be quite high. 16V rated part=3.2V, 450V rated part = 90V!
Electrolytic capacitors have a definite half-cell potential between the aluminium oxide and electrolyte.
If a circuit has resistances such as bleeder resistors that discharges the cap to 0V then the oxide is for sure depleted in time, usually over year and reforming is necessary.
Sitting on the shelf, I've had some in storage for years and a voltmeter gives 0.2-1V and I believe the oxide remains intact. But that's my theory.
The original oxide layer is created applying 140%-200% of rated voltage to a factory new capacitor, so adding oxide seems verses reforming the original oxide, I don't fully understand the difference.