the article uses science to come to arbitrary conclusions. some of their claims have no scientific evidence, but , for example, something being perceived as brighter is a good thing , it highlights it, it's not seen as a threat. corners are attractors for the visual system, and gradients give the perception of depth. Those are useful properties for UI design as they reduce the latency of our visual system by hundreds of milliseconds. I don't know if round corners have any real use