The other driver is ignorant of the rules and regulations, and as is usually the case when someone is ignorant of the
actual rules and regulations and their applicability, and instead are relying on flawed deductive reasoning, he is wrong.
The DOT FMCSA regulations apply to
Commercial Motor Vehicles (CMV), not merely Commercial Vehicles (A.K.A. cargo vans engaged in commerce of any kind). The DOT defines quite specifically which vehicles come under its regulations.
Definitions. - Federal Motor Carrier Safety Administration
The only part of the definitions that apply to a cargo van is (4) when the van is placarded for HAZMAT. At all other times a cargo van, even one hauling general commodity, is not a Commercial Motor Vehicle.
The DOT FMCSA is about safety of the public, and they are only concerned about the weight of a vehicle and how dangerous it is, or about what a vehicle is hauling with respect to HAZMAT (which is inherently dangerous). The hauling of general freight (commodities or otherwise) is irrelevant to the safety of the public, which is why the DOT could care less about cargo vans (unless they are hauling placardable HAZMAT, of course, because it is at that point where the van becomes a danger to the public).