With SwiftUI and Kotlin Jetpack Compose you can write architecturally and syntactically similar code across the two platforms. Essentially the same compact classes with the differences just being view property differences and OS API discrepancies (location, etc). So it's become a lot easier to manage separate development while keeping design direction and architecture on the same page. This is more so the case for apps with simple aesthetics and simple user interactions like Threads. It's not necessarily a sign that they are dropping support for React Native.

I don’t think they’ve ever really built entire apps in React Native outside of maybe messenger? Usually a few screens are implemented in it and the rest native. Or maybe features are prototyped in it and later rewritten.

They actually did another article with some good history on the iOS app for Facebook: https://engineering.fb.com/2023/02/06/ios/facebook-ios-app-a...

It means you get many more promotion points for creating a new framework and shipping it in a new product.

Also, generally internal tools are exceedingly poorly documented and supported so it seems easier to write your things from scratch.