I think what you're seeing in the pic is light duty panel adhesive, not filler.
I'd also hope that the body shop would take the bed off the truck, trim the joint to be a butted joint with a 1/32nd-1/16th gap, then use plastic based filler on the seam and a flexible backing on the inside of the butted seam. The best way to create a clean seam is to lay one panel over the other, then cut them at the same time with a 1/16th or 1/32nd wizzer wheel.
Betcha that's what happens at the body shop.
I'd doubt that ADD has branched into body work, so it's most likely a "mechanic style" bedside install to get them on the truck and the truck out the door to a shop that specializes in such things.
