You are right and therefore it should be easy for you to understand my point.

Both HTTP 2 and protocol buffers can be used with REST-like APIs as well. And technically speaking gRPC is just a POST request.

It would have been more useful to take JSON over HTTP 2 as a baseline, or at least include it, and maybe include protobuf over HTTP 2 without gRPC as well.

IMHO the more important point to consider is the architectural style. gRPC is the new SOAP.