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I don't believe this was claimed by the React team. In fact, I distinctly remember Tom emphasizing that they're going for "learn-once, write-anywhere" rather than chasing the pipe dream of "write-once, run-anywhere".

React Native provides the React way of managing the UI and provides a bridge to Objective-C (and Java, once they've released the Android version). It doesn't provide JS bindings to all of the host APIs in a cross-platform way, so you can't just build a full featured app in JS and run it elsewhere.

Plus, the React people stress that you should use custom UI on each platform. The key is that you get to build the UI using the techniques that work so well for the web.

With React Native you still need to write the UI for each platform. But you can share the models/business logic between platforms because it's all Javascript.