It does, but that's an unrelated issue to the one addressed in GP, which addresses the condition where the information exists on the server side when the response is sent to know what subresources are part of the composite that is desired.
> I'm not sure there are any popular, really REST APIs out there.
The Web itself is a popular API, which provided both the motivation for defining REST and which was, itself, shaped by REST in that REST was developed in parallel with and influenced the design of HTTP/1.1.