That sounds pretty reasonable, actually. Sure, you can make the case that if 40% of the apps are down, then that's a whole-service downtime, and should be counted as such. But what if it's 4%? Or 1%, or 20 apps? Should a downtime that took down 20 apps be counted at the same level as one that took down 80% of all of their apps?

Seems like the best compromise is exactly what they did. The numbers are about as relevant to people looking to serve their applications as they could be without breaking things down by availability zone or stack or some other metric.