3rd party apps still work in 120hz to an extent.
See excerpt from 9 to 5 mac.....
As it stands with iOS 15.0, ProMotion takes effect in third-party apps when scrolling or doing full-screen transitions. That means that you will still get a very fluid and responsive experience when scrolling your Twitter timeline, for instance.
However, almost all other animations are capped at 60Hz in third-party apps. This includes special effects and animations for custom components. It can be particularly jarring for the same third-party app to feature smooth scrolling but then less-smooth interaction experience elsewhere.