Considering they have never done that pre release why would they do such a thing? Updating apps to fit screen isn’t a 5 minute jobNo it's not. Test versions of iOS are released well before the phones and I would expect XCode to also be updated to support the new screen sizes in its emulator. So even without having the actual device developers should be able to test that their software looks right on the new device. Barring any visual anomalies, if the app looks fine blown up slightly larger on the XS Max then that can be perfectly fine from the developer's point of view. They might have far more pressing issues in their pipeline to bother updating the app for a new phone model which is not used by majority of the users.
Both iOS and Android support various systems for rendering application layouts and some scale automatically while some require tweaking for phone models because they are more exacting about size.