What do you expect them to respond? It's not very well defined.
I would probably say...
"""
REST, like many great religious philosophies, combines with one's life experience to mean something different for every pair of eyes that looks at it. For me, it's something which has URLs for resources of the form
/<class_name>(/<instance_id>(/<custom_method>)?)?
and default HTTP methods of GET / POST / DELETE to read / modify / remove instances. Finally, while I realize this is a bit too implementation detail-y than most definitions, my mind is not really at rest unless an HTTP API respond with JSON and reference other instances in some standardized way.
You will notice the repetition of sets of 3, and that's because 3 is my favorite number. I also really like the Stripe API's way of specifying arrows to related content to "unfold" references in the JSON response. That really puts my mind at rest when I see it, so that's RESTful, as well.
To get a little more real, though, REST is totally the old hotness and verging on not even being that hot. All the cool kids now know to use GraphQL, and if you aren't at least planning to transition to it, I'm not really interested in working here unless you pay me a lot of money.
"""
Well, strictly speaking if your having to explain the structure of your URLs then it's not really fully REST (because of HATEOS) and I don't think there is any requirement for a RESTful interface to return JSON.
But having said that I've got the point where I don't really care - if I can interact with an API from curl without too much difficulty I don't care if the API is fully REST or not.
If they say it is a set of guidelines and not a guaranteed, deterministic, contractual architecture like SOAP and there are many ways to interpret the meaning of "restful"; do they pass your interview?
