The rock is shown in realistic perspective, indicating depth. However, the authors presume that we are looking at the exact front of a flat object, such as an

The rock is shown in realistic perspective, indicating depth. However, the authors presume that we are looking at the exact front of a flat object, such as an axe blade. The realistic perspective disqualifies that presumption. If the rock was flat and blade-like, the painter could have shown this explicitly by painting the rock at an angle, like the corner of the wall behind them men.