As mentioned in a sibling comment, there are a number of scenarios where the people are fine but all lines of communications are down, which is functionally equivalent to everyone being killed:
https://news.ycombinator.com/item?id=16590441
(The actual wording on the e-mail is usually far more colorful, and usually involves a zombie apocalypse. Mountain View employees aren't just dead, they're undead.)
I think the assumption is that if the event actually did involve a danger to life & limb, response would be in the hands of Google Physical Security, local first responders, CalFire, FEMA, etc. Those organizations all do drill for rescuing people and treating injured, and will be in a much better position to coordinate responses than Google ops people.