I don't think it's a joke at all. One of the reasons Tesla is able to get the range that they do is because their car designs are aerodynamic. Normally Tesla does this by making their vehicles rounded off and pretty, but Trucks culture is very macho, with huge chromed grills that are completely antithetical to aerodynamic design. Tesla needed to cover the bed. They needed to make it essentially a teardrop design. With this they were able to sneak a design that actually ought to be quite aerodynamic but will appeal in a blade runner, retro-futurist way.