That grand cherokee is only 500 lbs lighter (9%) than my 4x4 2500HD ext cab 6.0L. With much more low end torque!
He has air bags in the rear, to allow for proper tongue weight, WD hitch, and double braked axles w/ controller. About as well setup as you can be to tow serious loads, with anything short of an HD pickup truck. From his business venture I presume he is typically towing 3000 lb, aerodynamic customer cars on that trailer and I'm sure it does just fine for that. This load really isn't much different aside from being less aerodynamic. No need to be rude; it's a perfectly legal setup and if he says it tows smooth and stable at 70 mph, why would you need to doubt him?