I admittedly don't know everything that goes on at Google, but last I heard, a PM would never be people-managing an engineer (although there's a separate "manager" track that sometimes ends up managing engineers), and all PM hiring required a CS degree or engineering background.

There was a period of time (roughly 2004-2007) where Google hired a bunch of managers without engineering backgrounds, but they discontinued that practice a long time ago because they found it doesn't work so hot. I've heard that many of the worst offenders have since attritioned away.