The way the owners manual reads it is, or should be to your average person, very apparent that the vehicle is the limiting factor and not the hitch. Since passengers greatly effec the published tow rating I would guess it is a suspension issue, which is also inline with the magnum issue.
The magnum has more rear cargo potential so it stands to reason it has a better suspension option, and thus a higher tow rating.
This isn't like slapping a Class V receiver on a HD pickup in place of a Class IV one. The vehicle limit is already lower than the hitch limit off the showroom floor.
I also read a few things where WDH is not supposed to be used on the Charger due to how the hitch mounts.
As mentioned the frontal area is going to be exceeded by anything but a pop-up, and exceeding that number will damage the drivetrain. It could be a 100lb trailer but too much surface area will still put too much strain on the vehicle.
Small example. I can pull a 15K lb dump trailer easier at 55mph on level ground than my 7Klb TT. Why? frontal area.