Mine were 14.5" , G rated, I think. That puts it somewhere around 3,000 to 3,500 lbs each tire. I can check the actual tires if you want.
However, for a set of 4 tires, that puts it in excess of 12,000 lbs.
I may try to add a third axle, so if I blow a tire, I'll still be able to tow to a safe pullout.
As mentioned, so far I've only done the aluminum boxes that are somewhere around 6K to 7K.
I do use equalizer bars.
As far as the other vehicle, those looked like 5 or 6 lug standard wheels. So, my guess is they were pushing the weight rating, although the tires didn't look flattened.
I would like a 20' or 24' steel box. Although I built my dolly/tongue to only haul 8' boxes, so if I find an 8'6" box, I'll be out of luck. But, loading and unloading a 20 footer will be different.