As far as I understand, the point of the article is to simulate a working backend in order to concentrate only in the frontend, the choice of AngularJS is contingent, you could do it with another framework (or none) but they chose this one. They aren't saying "Hey, Angular is the best for prototyping" but "Hey, not worrying about the backend is great for prototyping, and we do it in this way with Angular".
The interesting part is that you don't have to worry about web servers, server software, databases, scalability or server deployment; just simulate what kind of responses you'd get with the complete web application and iterate from there until you are happy with your frontend. This is particularly useful if your development process is driven by UX.
The interesting part is that you don't have to worry about web servers, server software, databases, scalability or server deployment; just simulate what kind of responses you'd get with the complete web application and iterate from there until you are happy with your frontend. This is particularly useful if your development process is driven by UX.