I think "ensuring humanity's survival" is more of a reference to protecting against cataclysmic events on Earth that might wipe us out. Some examples might be a nuclear holocaust, a deadly pandemic/epidemic, an asteroid impact, eruption of a supervolcano, etc.

Can you guarantee that none of these will happen in the next 50, 100, or 1,000 years? It's impossible to predict or protect against everything. Becoming a multi-planet species is the only way to ensure we survive in the long term.

There is a neat space-related DVD documentary that covers a lot of the risks that we face. It's called Gaia Selene: Saving the Earth by Colonizing the Moon. Might be worth checking out. The website is http://www.gaiaselene.com/. You can also find it on Amazon.