100% - frequently, my only path to giving someone a significant raise is via promotion. They may not want the next title, they may not want to manage, but I’ve often been constrained in being able to offer a better IC title, or to just pay a high performing Senior more.
It was a point of emphasis for me in my last search - commitment to, and belief in, IC tracks.
Quite odd that the last time around he complained about only getting a promo out of it, and this time it is about not getting one. Are we supposed to believe that he was promised a promo skipping past levels for delivering this? Who would even had the power to credibly promise that at a company like MS? Certainly not his VP, if the first story is to be believed.
Contrast that to if the guy had created a startup which made Copilot... he would have had the ability to benefit much more, but would have been exposed to more risk if it failed.
If you're dealing in expected-value terms, it's almost always the right move to just work for someone else.