Just looked at the flux presentation from facebook, and it seems to me that it's basically a set of best practice on how to wire your view actions to your controller's model and refresh. Having a single unidirectionnal "loop" model=>view=>action=>model... is something that i have been doing inside my angular controllers for a while...
Now react may help with the performances of refreshing the dom in that scenario, but so far i didn't encounter that issue myself.
I think the performance issues tend to come when you have a lot of controls and associated bindings via angular on a page. The use of jQuery (improperly) tends to exacerbate the issue. Most people won't come across the issue, to be honest. Facebook has a lot of points of display and interaction on their pages, so it quickly becomes an issue. Not to mention the simpler flow is easier to track.
I do find that the flux model makes more sense to me. Flux is a set of best practices, they use React for the rendering/view portion, and since their talks there have been several implementations of the workflow. Some using Ampersand or Backbone, others implementing their own pieces. Now Facebook has been releasing their own implementation...
It's all pretty interesting.
You are correct that React alone won't replace Angular, but with other tools (Ampersand, Flux, etc) you can use the pieces that you want and put them together. It's the difference between a library and a framework.