I agree with that, bringing rails in is like bringing a chainsaw to cut a twig sometimes...
The point here is not to look at the devise/router serving http but the porting of the data & business logic to an adaptable, multi-purposed api that is not tied down to the infrastructure in any way.
I've used this approach within a Sinatra driven app, CTO said i need to use Rails now for some other thing, a few hours to make the switch, not rewrite the whole thing.
As an employer too, the pricing model is super.