I'm a little disappointed they didn't go purely React Native here. It would have been a great showcase as they ultimately own the React Native project.
The app's target consumer is advertisers, and the app is this way because of those advertisers and their concerns. I'm not sure the person using the app is a "consumer" from Meta's point of view, more like a "digital subscriber."
However, the RN architecture does make it incredibly easy to bind to components written in native code - or use react native components from within native code
It was never Meta’s intention that something would not contain app-specific native code