What’s the Difference Between a Web Designer and Web Developer?

I think that at least some of this is historical based on how the capabilities in the platform developed. For example, HTML is declarative and, in theory at least, something all but the purest artist/designer understand. The same is generally true about other traditional communication mediums - Designers for magazines understand magazine printing and often have related tools, for example.

Likewise, CSS is not only declarative, but a rules based DSL for presentation - definitely in the realm of designers (and conceptually an underpinning upon which you could build even simpler tooling).

Where this has broken down and caused the kind of confusion i think the article discusses IMO is that the creation of HTML is very frequently (almost always traditionally) assembled via another system which is less so - asp/jsp for example were Turing complete languages mixed with templating. So it took engineers to make the dom serialization in the first place. Then as things moved client side we had script managing the DOM too. Combined with very limited semantic/expressive powers in HTML and CSS, the marriage between the three often put the two disciplines at odds with one another. I think that new tags, web components/shadow dom combined with new capabilities in CSS like regions, grids and pseudo-elements may put things right and make the web have a much better story for cooperating disciplines of engineering and design/ux.