From the README: "The architecture is a simple MVC heavily dependent on RxJS" and that's exactly how it looks, just linking different components together without hardly any glue code.
If all his code looks like this, he certainly doesn't need it to be in CoffeeScript.
I'm glad I don't have to deal anymore with people who irrationally cite such "articles" as argument fodder. They exist, in large numbers. But not on my team.
I've found that coffeescript prevents whole classes of bugs, and makes lots of javascript warts nicer to live with.
For example, you don't seem to run into scope issues, and it's a lot easier to bind this to what you expect.
5% more code (if it really amounts to that) is still more code, too! No code is like no code.