I think this post is at least partially misunderstanding the assertion that designers should be able to code. The point is not that they should be a coder, just that they should, like you said, "deeply understand the constraints of [their field]."

Someone who comes up with a good design that can't or won't be implemented may as well have not designed it in the first place- but if they understand the constraints on their design they can come up with something optimal that can actually exist.