Firstly, different type of truck in that video, so not a realistic comparison.
Secondly, that amount of movement between the cab and box is nothing.
Here is a much better example of what sort of flexing you can expect when going off road in a Fuso.
I should add... this is "normal flexing", as the chassis on these trucks is part of the suspension. It is designed to flex.
View attachment 408754 View attachment 408755
If this was a hard mounted to the chassis setup the majority of this flex would go into the camper box and there would be a lot of stress on the chassis between the front of the box and the back of the cab.
As already mentioned here, a hard mounted camper box is not ideal if you plan on going off road with the truck.
Hopefully Westyss does not mind me using some images of his truck in action to make my point.