A very good explaination, IMO. It only highlights that a capacitor is just a bandaid for a weak power source.
You are correct but I would never use a cap to fix a poor power source. A cap isn't even a bandaid for a weak power source it is designed to enhance a strong power source.
Now lets get a little technical.
Lets assume a 1500w rms class d amp has an efficiency rating of 80% and that you charging system maintains a 13v output.
1500/13=115 amps at 100% efficiency
115/.80=143 amps at 80% efficiency
This means at the rms rating on the amp it will require 143 amps to produce 1500w at 13v input (i Know this excludes other factors but that would be a little too technical for the sake of this argument)
Now lets assume this same amp has a max power rating of 2500w. Which means it can produce this much power for a very short time(again realistically you will never see this amount of output but again for the sake of argument)
2500/13=192 amps at 100% efficiency
192/.80=240 amps at 80% efficiency
The difference in power requirement at 80% efficiency between rms and peak is 97 amps while you would want to put an alternator in that would handle the additional load it would still pull a huge draw on the charging system having a cap in place allows that additional 97 amps to be drawn from the system when the amp no longer needs that power therefore the amp won't "see" that system draw and will run closer to the 13v at all times. As stated many times a cap can't create extra power but it can store and release that power at the peak load situations then in turn recharge when the system load is relatively low thus spreading out or delaying the power need from the charging system.
On competition systems the reason that batteries are used instead of caps is they need to produce power long enough for the subs to produce max spl. A cap would not give a long enough burst of power to get a sufficient reading.