He made his PhD at Stanford in the nineties and had a company with Bak and others in Palo Alto and still lives there. So no, I don't think that was the reason. And apparently Google values its Zurich location so much that it is constantly being expanded, see e.g. https://www.blick.ch/schweiz/15-jahre-in-der-schweiz-so-sieh.... So obviously it's not just an office.