you posted a link comfirming he did create the immediate mode ui concept, so I don't see your point. I never claimed he came up with "immediate mode" term from graphics programming.
I would also disagree that game scene are simpler than GUIs. game scenes just need to pump triangles, colors and textures. gpus are great at that.
tesselating a GUI is much more complicated, and you need to be smart about texture atlases for glyphs and images.
maybe immediate mode *graphics* programming was slow, but modern IMUI stacks like egui use modern graphics apis like wgpu/vulkan/metal and are quite fast.
notedeck and damus android will be the fastest and smoothest GUI app on any device you throw at it, even $100 android phones. I think this is proof enough that repainting everything is faster than retained mode diffs.