The stiffening capacitors are probably a combination of various sizes of capacitor, which would increase the self resonant frequency up a lot. It would certainly improve the high frequency response.
The car battery will supply 1000A or so, but the battery voltage will be dipping, and if an amplifier is far from the battery, a big capacitor is a good idea.
It is always good practice to add a capacitor near the point where current is used, especially if the load varies. Car amplifiers are no exception.
Having said that, anything can be badly engineered.