The library it is using is only available when using web technology, so it’ll work in hybrid and responsive mode but not Native: https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesis.
You’d have to use a different React Native library to achieve the same functionality. Something like this:
I hope this helps? I agree the documentation should have a warning. If you submit a feedback, i’m sure someone will pick it up and change it for you.