Ooh, *plausible* snake oil: the worst kind! A big capacitor in parallel with a battery can in principle help; but I have to doubt that this one would make any detectable difference to the Prius 12V battery, which has very gentle demands placed on it.
It might help the audio system if you have amps that might have large surge current demands, as long as the average current demand doesn't exceed the existing supply.
What this device appears to be doing is trying to compensate for inherent internal resistance within the battery, as well as voltage/current fluctuations that would occur with a standard alternator running at changing RPMs.
Considering that there is already a solid state 12-14V power supply replacing the alternator: the DC/DC converter that is part of the inverter assembly, the 12V supply line should be quite stable as it is. Fluctuations then could only be caused by large fluctuating current draws over enough wiring resistance, which would show up as voltage fluctuations at the point of draw, not the point of source.