The actual quote is "In fact, in places where we’ve been able to reopen Google offices in a voluntary capacity, we’ve seen nearly 60% of Googlers choosing to come back to the office." That doesn't sound like surveys; I think the 60% of people came to the office once or twice interpretation is more likely.

(Also possible that the offices that were opened were more heavy on hardware / "lab" roles, so while maybe it was officially "voluntary", those people effectively had to choose between coming back to the office or doing fake / busy work at home.)