Yes this is correct and a usefull application which makes sense, particularly if there is an unusually long supply cable run, of limited cross section, e.g. battery to trunk.
An audio amplifier will draw high current at high sound peaks in the signal and the resistance in the supply leads will result in voltage drop. Fluctuations will be of a very short duration and therefor a large capacitor is capable of "filling in" such short depressions in the supply voltage. That said, if the supply line is adequate, an extra capacitor will serve no purpose. Any amp of sound design will incorporate existing built in capacitors, to cater for what can be considered normal.
The same theory can of course be dishonestly claimed in respect of the ignition system, but the same conditions do not apply and the sales pitch made is invalid. Looks good, a talking point, fun to fit, costs you, surely must feel/work better.
