All correct. It's nice that they provide those instructions, but for the vast majority of US installations, they are useless.
It does tell you that the scanner has it's chassis isolated from power ground though, which is interesting.
As has been noted, just wire it negative ground. Positive ground causes more chassis corrosion and that's why it was abandoned. You may have noticed over the years that the positive terminal of a battery tends to corrode more than the negative in an atmospheric environment.
Similar things are actually done using various metals on metal ship hulls using "sacrificial anodes" that get worn away to minimize corrosion of the hull plates.
Here's a wiki on that if you're interested: http://en.wikipedia.org/wiki/Sacrificial_anode