From the comments section of the article…

    “At MacHack, when most of the original team was there and
    Andy told this story, Jef Raskin spoke up to say that he
    had told Steve about round rectangles being everywhere,
    and that's what got Steve obsessed about them.

    But what I found interesting was something Bill Atkinson
    said about creating the RoundRect routines. He said they
    turned out to be a lot easier to create than he thought
    they would be. I can't tell you how many times I've
    over-estimated something easy. I think it's something
    inherent in computer work.”