It's not that hard. In India because of so much construction related digging cuts OFCs, we do the path planning quite well and our redundancies get tested quite regularly whether you want to or not.
But Google's vendors might have less. One would hope that Google is auditing claims of independence from vendors at least somewhat, but at some level they have to rely on vendor representation and SLAs if they aren't going to do it all themselves.
Accidents happen. Regularly. :D