The main thing that comes to mind is why they do not deploy these kind of changes to a small slice and smoke test the slice before deploying to all users? This seems to be a pretty common routine for services at scale nowdays...?
I suppose this is a hard problem to solve on a configuration change level though. Imagine the configuration change that triggered the bug was something like "hey load balancers, stop sending traffic to the cluster with that new version of service X which seems to cause elevated error rates." You don't really want that kind of change to take too long to propagate.