Carmack didn't "sell out", he spent decades at id and was responsible for vast swathes of 3D technology and the open-sourcing of multiple cutting edge game engines.

He went above and beyond what you could expect of anyone who cofounds a game development house and he is treated like a literal programming icon for good reason.

In this case it's more a polite way of saying "the founders got forced out" - or more bluntly: the idiots in charge managed to get rid of id's greatest asset.