I love this article. It’s not often you see designers advocating for developers. I think designers should do what they should already know how to do well: step into the developer's shoes for one moment and understand the things that make developers’ lives difficult, and try to make their lives less difficult.
Like you said, there are overlaps amongst all these positions. For instance, often times css/html is a job requirement for UX designers. We don’t need to learn how to code, but it’s important for us to understand the logic behind the code so we can work well with developers to make a product better, with a lessened load time for end users, and in general work more efficiently.
The issue w constant and close collaboration is that sometimes/realistically there may not be enough time for developers to sit down w designers because they also only have so much time to do their job. I think that’s where you hit the nail on the head about talking about prioritization. Maybe it would help if across the board teams could think about building this collaboration time into their sprint schedules to make things easier for everyone.