Right, because you don't double up the two front wheels which are used for steering. I suspect that more weight is put on the rear and trailer axles than the front as well, and that two tires in close proximity aren't as good as two tires separated by greater distance. So even 1000x is probably optimistic.

My main point though is just that the study happened to use truck axles as a measurement and that can't blindly be substituted for car axles. The back of the napkin math is just there to show the difference it can potentially make.

Also, truck tires are also much larger (than an average passenger car) and have a larger contact area.

Passenger off-road vehicles with knobby tires might be harder on roads too in a different way than weight.

I actually thought this was the case but a quick search indicated about twelve inches wide for both. Was I missing something?

It would make a significant difference to the calculations, eg 50% wider tires would mean dividing truck severity by 5.