Maybe they need better immediate mode libraries, then. Repainting everything is definitely not fast as repaiting just what's needed, even in today's hardware.
Also, he did not coined the term:
"Excited about the results, I [Casey] reported on the technique to a private mailing list in the fall of 2002. To describe it, I coined the term “Single-path Immediate Mode Graphical User Interface,” borrowing the “immediate mode” term from graphics programming to illustrate the difference in API design from traditional GUI toolkits. I continued to use the technique for several years, until I finally put up a super rough public video describing the basics in 2005".
Immediate mode is has been a term in computer graphics long before him.
GUIs are 100x simpler than game Scenes, so it makes sense to apply immediate mode to GUI design (like the old graphics people used to do), but the GUI itself has severe upperbound limits on what it can do.