Yeah this is kinda the big elephant in the room here? They didn't prove what they set out to prove. Yes obviously OpenGL does scaling just fine, the entire point of Wayland is to get the compositor to just being a compositor. They didn't do any scaling with X. They didn't do anything at all with X other than ask it some basic display information.

Toolkits don't use X to do much (if any) drawing these days. They all use something like cairo or skia or -- yes -- OpenGL to render offscreen, and then upload to X for display (or in the case of OpenGL, they can also do direct rendering).