The take-away from the article is that a real syntax extension such as `a[1:3]` is not possible in the confines of ES6 because the language designers don't know

The take-away from the article is that a real syntax extension such as `a[1:3]` is not possible in the confines of ES6 because the language designers don't know why it's good to have that capability. You have to resort to external work-arounds like babel, sweet and others but they do not work in all circumstances and are all incompatible with each other.