"No doubt"? Really? He's been there 2.5 years which is longer than most founders stay on after acquisition. It's certainly possible he was fired. Maybe even likely. But I certainly have some doubt.
He's an "advisor". He has neither moved on to a more prestigious role in Google/Alphabet, nor has he started a new company (that he's announcing publicly), nor has he moved to a prestigious position elsewhere.
It's clear that he's been pushed out.
Curious why he would need to move to a more prestigious position? Most people realize by their 30s that prestige is a sucker's game; it's a way of inducing people to do things that aren't much fun and they wouldn't really want to do on their own, by lauding them with accolades from people they don't really care about.
Lots of people quit jobs and take some time before doing their next thing.