Well, let's be clear here. I use React Native, and it would take me about 30m to write a bridged React Native method that could execute ObjC code dynamically, including accessing all the private APIs you could want.
Yeah. Objective-C is definitely really flexible. From my reading of the warning, the important thing is not to execute Objective-C dynamically or access private APIs regardless of whether you're using React Native, Cordova, a WebView, or even a bare iOS app.