My guess is a little of both. I would bet that the 1% is the unused cost, but that for periods that they are running the stand-in system it's much closer to normal. But from what I've heard of their infrastructure I'd also bet that stand-in is significantly cheaper in part because it had the benefit of hindsight and could resolve issues in the first system.
None of this really changes my original feeling though. A cell based architecture spreads the risk, failover is essentially built-in without being a special mode you have to put the system into and then recover from again, on cloud you can scale those up and down with traffic and they also take peaks from each other, and so on. There's a reason why there are very few cloud outages that cross all regions for a provider, and if you're paranoid you can run multi-cloud (as they already do).