I've been a Tesla owner since 2015, and an update has never been like this.
In previous times it has followed exactly like you said. A few releases of small sizes followed by much larger releases. This time there is so much of an emphasis on California that it's indicative of something else going on.
As of this writing around 4.4% of the entire TeslaFi Model 3 fleet has 2019.5.4. But, the TeslaFI fleet has cars that exist outside of the US and I believe the release is limited to the US for now. I'm assuming the Canadians complaining about the lack of Sentry mode is correct.
So that means the percentage is actually much, much higher for California. I tried figuring out how many California Model 3 cars are active on TeslaFI, but I can't find it.
But, we can reasonably ballpark it at around 25% as Tesla sold more Model 3's in California than anywhere else. So that means Tesla has around a 20% install rate of Sentry Mode in California. That's an awful lot of cars for a software development effort (to look for bugs, etc).
It's too early to tell if its been officially released in California (in an incomplete form) or they simply needed a LARGE sample size to get adequate measurement.
Having so many means Tesla is risking people having alarms going off, and the car getting a bad reputation. If there isn't enough then they under sample. I think it's important to emphasize that this feature is likely nowhere near as easy to implement (correctly) as it might seem. Like right now I believe you have to pull the stick to review footage if the alarm goes off. So at some point the video sample needs to be part of the push notification. Like my porch security camera does when it detects a human in my backyard.