It might be more accurate to say that he repopularized the term among a new generation of developers. Immediate vs Retained mode UI was just as much a thing in early GUIs.

It was a swinging pendulum. At first everything was immediate mode because video RAM was very scarce. Initially there was only enough VRAM for the frame buffer, and hardly any system RAM to spare. But once both categories of RAM started growing, there was a movement to switch to retained mode UI frameworks. It wasn’t until the early 00’s that GPUs and SIMD extensions tipped the scales in the other direction - it was faster to just re-render as needed rather than track all these cached UI buffers, and allowed for dynamic UI motifs “for free.”

My graying beard is showing though, as I did some gave dev in the late 90’s on 3Dfx hardware, and learned UI programming on Win95 and System 7.6. Get off my lawn.