The logic of bureaucrats is often so skewed from the rational that one cannot see the path they've followed to get there. It seems obvious that regardless the capacity of the chassis, if the bodyworks mounted weigh less, and there is not cargo space built to carry freight loads then the vehicle rating follows the load carrying capacity that can be accommodated, in this case a few passengers and their provisions. To go by the chassis capacity would lead one to say that if GM used that chassis for multiple applications, say from "heavy goods" to pickup trucks to city buses they would all be classed to the chassis max. I sympathize with your situation and hope that you get a nice Christmas surprise of a new certification in your stocking, if not before.