All very true. Only addition would be that the battery while like a capacitor is like a really crappy high ESR capacitor. So unlike a ceramic or something that takes away all the high frequencies, it will eat all the low frequencies. For car audio, that is a great thing. If you are driving through a giant EMF it won't help though. :)
In fact you write as I do - both my audio (a standard 4 x 45W HU) and aux battery have their grounds cabled to the battery -ve. (The aux battery mainly because I was using telco fig-8 (2-wire) cable, and the isolation relay is mounted on the side of the main battery.)
But no argument from me - the "quietest" electrical connection is back to the battery -ve. That's usually because the battery is a big capacitor - it is the battery that filters (out) the alternator AC ripple as well as other noise.
(The exception is where ground-loops occur due to high-current voltage drops along distribution cables, hence the need for a more local single grounding point. But that is more for big audio systems (many tens of Amps) and where audio-noise susceptible signal grounds are involved. That is generally irrelevant to PCs etc except where they too have similar analog ground issues (sensors, audio, etc).
My original reply was a reflection of some trivial else stupid unqualified claims I have seen elsewhere.
But these days - with lighter panels, composite materials etc - the dedicated wire being less resistance than the car chassis is becoming more and more relevant and true.
(I know cables have a lower resistance than fiberglass boat hulls.... LOL!)