This article is plain non sequitur. The argument stated is that intelligence is multidimensional (stated as fact with caricature testimony) and therefore there exists a job aligned with one of those dimensions. Arguing that designers should code states that today optimal design is done by someone who has skill on multiple axes. So the article really just dodges the issue by defining situations alternative to argument…
That point is underlying the whole article. ;) What it tries to clarify - and well, it might have failed, of course - is that we should stop to simplify the problem as "designers should code", because not all designers should . But yes, all designers could , and if you feel that way, or you are prepared for that, or if you want that, you can learn to code. Exactly like a developer can learn how to design. In either c…
A designer is someone who designs—it's defined by action, not beliefs about competency. If you design things implemented in HTML/CSS and you don't have any proficiency at all in those technologies then you're very liable to be a worse designer than someone who does. The argument of how often that is true is what's important here and it's never actually discussed in the article.