Interesting FAQ entry. The transition from the second last to the last picture is a little confusing though (at least to me). Suddenly the vehicle frame pops up as the only conductor for the negative pole. I don't know much about ICE installation, but I was always under the impression that if you do put extra audio equipment into your car you have or should have exclusive conductors for both positive and negative poles from the device to the battery, hence grounding wouldn't be necessary and there shouldn't be any ground loop. But perhaps using two cables instead of one all the way from the battery to the device is just one of the measures to prevent a ground loop.

Either way, this should be elaborated on in the FAQ entry.

Georg