Cracking video, thanks for sharing.
As to the thickness, look at all the bulkheads throughout the section, it is like a cantilever bridge, everything supports everything else without having to have heavy plates which survive 15 inch naval gunfire.
The lighter it is, the more economical, the cheaper the tickets, so we can go on cruises for the same price as a reasonable hotel break (well, here is the UK anyway).
Andy