First of all, a unibody construction doesn't mean that the complete bed is part of the body, there could be parts on top of a foundation which can be easily removed. Also, steel means, it better repairable than any other car material. You can reasonably easy cut out parts and weld in new sheets. That most of the surfaces of the truck are flat makes this even easier.
You must have missed the part about how the steel is stainless. And also welding is extremely labor intensive, and time consuming. The flat body panels actually make this harder, because you can't warp the panels or else it will be extremely obvious. And if there's no paint, there's no filling in dents with body filler, it requires absolute precision, and metalworking skills which haven't been around for years in the auto body collision industry. I can't tell if it actually has paint or not, but you should look up how a small dent is fixed in a Delorean.
