It makes sense to me. The previous generations had pages that had more rendering on the server and "forms." Now, it's single page applications hitting REST end points.
So before, you had complex models, tightly bound back-end controllers, sending data to a view.
Now, you have simpler APIs on the server, you arrange and manage the data in the browser, getting more data as needed.
Rails is perfectly well suited to serve an API. In fact I believe it was the first to define off the shelf RESTful conventions for an entire controller and all verbs by default back ~2007 if I recall.
Actually, Rails 5 implements an API mode so you can spin up a new app with only API-level conventions that runs on less memory and just serves JSON. You can add on the standard Postgres DB and Boom! "You just made Rails great again"
