I'm sure there are gray areas in such contracts but something being down or not is pretty black and white.
This is so obviously not true that I'm not sure if you're even being serious.
Is the control panel being inaccessible for one region "down"? Is their DNS "down" if the edit API doesn't work, but existing records still get resolved? Is their reverse proxy service "down" if it's still proxying fine, just not caching assets?
Is it? Say you've got some big geographically distributed service doing some billions of requests per day with a background error rate of 0.0001%, what's your threshold for saying whether the service is up or down? Your error rate might go to 0.0002% because a particular customer has an issue so that customer would say it's down for them, but for all your other customers it would be working as normal.
it really isn't. We often have degraded performance for a portion of customers, or just down for customers of a small part of the service. It has basically never happened that our service is 100% down.