I'd say it's the other way around. Url's are opaque for the client - id's imply more knowledge of the implementation. E.g. the client would have to know which host to communicate with and how to structure url's from id's. With hyperlinked documents, all the client needs to know is http.
URLs are opaque, and can often serve as very useful IDs, but, alone, they imply a one-at-a-time model of fetching documents, and this spec is trying to provide a way to easily request only the documents a client needs in a compound document.
Keep in mind that this spec actually requires that every ID be able to be readily converted into a URL based on information found in the same payload, so URLs are still front-and-center in the design. It just separates out the notion of a unique identifier, so that it can be used in other kinds of requests.
