He doesn't have an AI background, although I'm sure he's picked up some stuff over the last few years. Google made him head of a bunch of different things because of his project management skills. (I used to work in his department.)
He founded Metaweb but unless I wasn't paying attention he doesn't have a PhD. He's the same age as me so his academic career was in the 80's. jg isn't a big time AI guy but he is very smart and he's been deeply involved with AI projects for at least the last decade.
KG is not what most people think of as AI. It's more of a very large triple database with some confidence metrics than anything like what would pass for machine intelligence today. It's not even in the same org at Google.
He doesn't have any modern AI experience. However, since researchers like Ruslan Salakhutdinov are already working for Apple, Apple already has the tech expertise, and probably wants people who can grow and manage teams, and get the most out of their AI talent, something researchers like Russ are not experts in.
However, I think this move benefits Google as well. Jeff Dean now leads AI, and unlike JG, Jeff has both the technical chops and the managerial expertise to boost Google's AI programs. No more having to approve every project by someone who doesn't understand the nuances and capabilities of modern AI. If Sundar gives creative freedom to Jeff, we might even have singularity by 2050. I am only half joking.
He's founded and managed Google Brain. That doesn't compare with someone who's more focussed on management, but I think only a handful of people in AI have both technical and managerial experience, and Jeff is one of them.