Is React Native the true native 'cross-platform' framework? Apple TV and Ubuntu forks say so

Let's recap, shall we?
It started with React Native for iOS, and then we got Android support soon after it.

You surely didn't miss React Native desktop for macOS and the plugin for Universal Windows Platform (UWP).

React Native plugin for Universal Windows Platform (UWP)</p>

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Heck, even Samsung is building React Native for its hybrid platform.
Now, what's left?
Oh, that's right.

react-native-appletv README on GitHub

This branch includes changes to support building React Native applications on Apple TV.

The changes have been implemented with the intention of supporting existing React Native iOS applications so that few or no changes are required in the Javascript code for the applications.

This branch includes a working version of the UIExplorer example project for Apple TV.

react-native for Ubuntu README on GitHub

There is support for applications on both Desktop Ubuntu and Ubuntu Touch.