Clearly it's a staggered rollout thing that hasn't fully taken effect. (Which is stupid, when you do this kind of thing at scale you pick an exact time to go "live", add some code that enables the flag at some point after that exact moment in time, then do a rollout early enough that all the machines in the cluster are updated with that code by the time rolls around. And then everything "just updates all at once." Why do I suspect that Google from 2006-2009 would have made the effort to do that for this update, but that nowadays it would have been deemed as too difficult or whatever...)
I wonder what happened. Maybe enabling this _now_ was a last-minute decision? Weird.