He delivered revenue growth by enshittifying Goog's products. Gemini is catching up because Demis is a boss and TPUs are a real competitive advantage.
You either attribute both good and bad things to the CEO, or dont. If enshittifying is CEO's fault, then so is Gemini's success.
Demis reports to Sundar. All of Demis's decisions would have been vetted by and either approved, rejected, or refined by Sundar. There's no way to actually distinguish how much of the value was from whom, unless you have inside info.