Making assumptions like this is a dangerous habit.
1. React and Ember are different things, one is a framework, the other is not.
2. Ember 2.0 was a backwards compatible evolution of 1.0 that shifted the approach from MVC (old state of the art) to a more components based approach (similar to React in concept, not in implementation). Tom Dale was instrumental in making this happen, he didn't just ship v1 then proceeded to use React while doing some sad Ember support on the sidelines.
3. Why would he be one of them? Angular 1.x was the best thing for many JS developers, yet React came along. Why wouldn't the React developers just love Angular 1? There's a possibility that React will loose popularity to another solution.
4. Popularity is one potential indicator of how good a solution is. Despite it's popularity, many seasoned JS developers heavily disliked Angular 1.x because it felt like an overengineered-java-like-approach to building web apps. There are many seasoned JS programmers who are moving away from React to other solutions (Angular 2, Ember) because they have other pain points now, than 2 years ago.
For example, Tom Dale once told me that he doesn't like redux as a solution for keeping the entire app state - yet Redux is all the rave now in Reactland.
5. JS (similar to PHP) has huge numbers of junior developers that can't build a web app on their own yet are working inside teams and are, in various degrees, competent at doing specific tasks. Almost all of the junior developers I know have heard of React and want to work with it despite not having the least understanding of what problems it solves and what problems it adds.