I believe that in many organizations, this premise is flat-out wrong. We absolutely need designers who can code - and the kind of "production-ready code" that the author mentions is not the same code that I expect designers to write. It's true that I might not trust someone who is a designer first and developer second with writing a front-end framework. But wiring up a static HTML page with some Angular directives, or even just some jQuery? I feel like to place that beyond the capability of a designer is incorrect, if not plainly insulting. Design itself is not a simple art, and anyone who can get good enough at it to be worth hiring as a designer is certainly capable of learning the bare minimum necessary to be self-sufficient on the front-end.

Designers who cannot code come with lots of hidden costs - if they want to prototype a complex interaction, perhaps with some animation, they need some developer time or are using tools so complex that they may as well just learn to write the code. If they produce a design that doesn't work as well on a phone as it does in Photoshop, that's a lot of lost developer time. In my opinion, these are costs that a lot of organizations can't afford, and don't think about when they hire purely visual designers. All that talk about "you can only be an expert at one thing", while true, misses the fundamental point that designers don't need to be expert developers: they just need to be good enough to enjoy the benefits of being able to bring a design into reality (regardless of whether that code actually makes it all the way to production).