>Since he is using the Cairo library everything is rendered locally and transferred in uncompressed form to the X server for drawing. The same is true for all font engines which use in the case of Xft the Xrender extension.

So this is not necessarily true, it depends on how you're using Cairo. If you're using Cairo's xlib surfaces, those will try to accelerate with XRender if that's available: https://cairographics.org/manual/cairo-XLib-Surfaces.html

As far as why the toolkits never got on board with it, some do actually use XRender in certain circumstances, but it's hard for all applications to make use of it properly. Most of the advantages go away if you have any images coming down the pipeline at all, or if you need to do any text layout or reading back from the pixmap, or if you have any GL/Vulkan usage in your application, etc. Plus historically I've heard the implementation of this was very inconsistent and on some servers it actually resulted in a slower rendering path, although this should not necessarily be the case if you're using the current Xorg with glamor enabled.