Border radius is not 'correct design'. It's subjective design. There are places where `border-radius: 0px` works and places where it doesn't. Context matters more in design.
On a side note, easy to compare by crossviewing.
Really though, some design aspects are objective, but some are not. The author of the page seemed to be relatively on point though about whats objective and whats not.
It's a known fact that typefaces that have curved, humane shapes are more legible. That is why it is bad practice to use something like futura for long text.
I am assuming rounded corners in boxes follow that logic.