I think you didn't catch the author's definition of an API: If the machine and the human can access the same resource on the same URI - it's one API. Don't need

I think you didn't catch the author's definition of an API: If the machine and the human can access the same resource on the same URI - it's one API. Don't need to convert the Json on the client side - it can be done on the server side whenever a client specifies that it wants the resource returned as application/json in the HTTP header.