It does look like a dinosaur. But fossils still existed in prehistoric times and would have been found by prehistoric peoples. Just because there's a carving that looks like a dinosaur doesn't mean that dinosaurs existed at the same time. Besides, stegosaurus is known to have died out before the cretaceous, so they were gone even before the extinction event.
Similarly, there's modern influence on ancient objects. There's a gargoyle of Darth Vader on a cathedral in the US, added later when they had to replace a broken gargoyle. Doesn't mean that Star Wars existed at the time the cathedral was built hundreds of years ago. The dino could easily have been photomanipulated or carved out at a later date.