This is sort of not right. That particular patent was a design patent. Not a utility patent. When people think about patents they usually think of “inventions.” That’s a utility patent, which is a different kind of patent. Design patents are more like copyright - they give you exclusive rights to the way something appears.
In this case the design patent was not for “rounded corners.” It was for the overall shape. When you get a design patent you provide a picture, and you are only entitled to what is in that picture. The round corners were certainly a primary distinguishing feature, but the whole “patent on round corners” argument is a gross mischaracterizstion both because it’s not accurate, and because most people don’t understand the difference between design and utility patents.