Uhh you described exactly the situation I referenced, just bunch of internet randos making guesses based off a picture, with zero access to the actual truck making arguments against Tesla engineers who actually engineered and built the truck.
Here is a video where the engineers talk about the exoskeleton providing significant stiffness to the structure.
If the outside skin provides a significant of structural ridigity, then by definition it is an exoskeleton. People probably had the false and idiotic notion that the inside of the truck would have no unibody. That would no make sense, since there would be no places to mount the suspension, drive components, and interior. It wouldn't have made sense to have an exoskeleton like a clam, hard shell and mush inside, clams don't move. Cybertruck is more like a exoskeleton like a crab, shell on the outside, but also structure on the inside too for attachments for locomotion. People also had the false notion that it should all be one continous structure, which also makes no sense. That is the definition of a monocoque, not an exoskeleton. Exoskeletons in nature in moving animals are piece meal, like on a crab or insect.