Its true that modern RDP has some extensions that do some of these things, like MS-RDPEGDI. I don't deny that, but as I've said, this discusson is not about RDP as-is. There was never any interest whatsoever in using exactly the RDP protocol in Wayland or X, that makes little sense as it is quite Win32 specific.
What I mean is that when Wayland developers say that "VNC" or "RDP" is better than X, they don't mean using the exact same bit streams, but rather the basic (original) concepts of these.
As to a protocol implementation, Kristian has an initial implementation of wayland remoting in his git repository and has been demoing it publically. I don't think its quite usable yet, but its far from in my imagination only. And even if there wasn't an implementation, I don't understand how discussing the real weaknesses of X vs a planned replacement is irrelevant?
Modern RDP is actually kind of a mix host and client rendering depending on what APIs the app uses and what kind of graphics it renders. If apps hit the right codepaths they get drawing commands piped to the client, otherwise it gets host rendered. Then they ensure that the most basic OS things like DWM gets this right so that e.g. window management always hits the efficient paths. In the case of Wayland the WM side is less problematic as the compositor always runs on the client, but for sure some of the more modern RDP extensions are interesting to look at. In particular I think forwarding of video streams is important to get right.