It sounds to me like some are speaking of the truck camper by itself rather than being secured to the truck. With the truck involved and you never know details of which vehicle it may be used on the truck camper mfgs couldnt address an "overall" COG. They could only speak of the camper only. The wheel base would be a factor in front to rear.
I dont know how to apply horizontal/vertical to this concern. Horizontal could mean front to rear or side to side - both referencing a tip angle from the horizon.
My concern would be side to side of the vehicle with the camper secured.