They do that without vinyl especially when the seals around the doors are bad. there is cheap plywood behind the thin metal and it gets wet and holds water. Typically the lower sides and bottom are the worst.
Looks like run of the mill corrosion. We see a good deal of it here on trucks that are not wrapped. Even the stainless trim corrodes down here at sea level. Unwrapped hardware on the truck looks like it is well used.
It is pretty hard to cut through the powder coat/baked finish during install, unless you had a very heavy-handed installer.
If that was the case there would be a lot more damage at most of the trim edges.
Tell him it might be Covid and see if he can claim it with the feds.
I'd point out the lack of failures above the bottom 6", have him show you what they find when they tear into it. If it's rotting wood on the panel, have him tell you where you must have poked a hole through the steel/aluminum skin. If it's rust, I'd assume it was previously wrapped, removed with the standard of degreaser and pressure washer, and blew away any hairlines slits in the paint, exposing it to oxidation and rust formation.
Assuming you installed this totally dry after cleaning, make sure he is aware of that.