Apples to oranges in many ways.
That rig is probably built using a cab/chassis base that utilizes a very stiff straight frame.
For the most part, chassis flex is pretty minimal. The suspension must do all the work.
The entire coach does look to be fiberglass as well, so some flexing would be perfectly fine.
The fiberglass if properly built will allow for some flex without damage.
With all that said however, flex is inevitable, especially with such a long vehicle.
So building a unit like that is quite a task if you want it to survive off road.