The specific part is configuration as code. So the config change (flag activation) and code change (flag calling) would have been synchronized.
And there wouldn't have been one server of 8 with a different build for a meaningful time and also if it did fail to deploy on that one server it would have been obvious.
That's based on the assumption that someone would have thought about testing that particular flag for that particular scenario.
In my view this would only have been caught by a deployment to an identical copy of production, with running, simulated transactions, and high level funtional testing. Testing for each individual config value and scenario of where it may be used is playing whack a mole. Basically, I'd make a clone of prod, simulate everything that happends externaly (APIs, etc) and observe transaction KPIs and other high level business indicators. Testing for tech is insuring that the tech works, and sometimes that means testing that it's broken.
The specific part is configuration as code. So the config change (flag activation) and code change (flag calling) would have been synchronized.
And there wouldn't have been one server of 8 with a different build for a meaningful time and also if it did fail to deploy on that one server it would have been obvious.