This is true but VNC doesn't exactly win any awards in the performance department in general. Sending bitmap diffs is pretty much the least efficient most naive way to do remote desktop. What VNC has going for it is that it's dead simple to implement because of that fact.

X forwarding goes the complete opposite direction to pipes the whole X protocol over the network. Convenient -- yes, also dead simple to implement -- yes, actually works -- well no. You see the X ecosystem also assumes all rendering is local, that fact is just hidden in edge cases. For example Nvidia graphics work on X with an extension that both has a client and server component -- as soon as you forward X traffic that's looking for the server-side extension... crash.

RDP by contrast does the right thing(tm) but also the hard thing by defining an intermediate object model which is semantic enough to kill the need to send pixels over the wire most of the time but flexible enough to allow for it when necessary.