I0540,
Sorry to break the news to you, but all this "old ICAO stuff" is quite new - the ICAO PBN manual being published just two years ago.
Sure, a GPS receiver can, under certain circumstances give an EPE of single metre accuracy, but RNP is about how a total system works; it's intergrity, redundancy and operational procedures. The technical standards have as much to do with rejecting reflected signals along with continuity of performance with terrrain and structure masking.
Bit more to it than meets the eye.