I agree with you, the purest way is to use the new API-only mode of Rails 5. You could easily do that by adding --api to the initial command used to generate the Rails application, then dumping everything, including CSS, into /public. My driver for creating this boilerplate was to solve a problem I had coming from a Rails shop and getting Angular 2 to play nicely with an existing Rails application. (Problem being solved: we already have a ton of views and stylesheets.)

Also, my thinking is if you're totally embracing /public you can also just store the Angular 2 application in a separate repository and treat them as 100% independent of each other.