How is it a backhanded compliment? The React team is obviously proud of their work, and used it as a point of reference that they believe they can do better than what is there now too.
They're just acknowledging that everything is moving forwards, I imagine that they're pleased to have influenced the other frameworks. Ember in particular seem to have learned a lot from React and are adopting a virtual-DOM approach to rendering.
Considering React says that there are things they can do to improve React even more, suggested to me that React is currently as fast or slower than those other frameworks, but React can once again become faster than them.
i can't be sure about Angular but they directly reference Ember, who published recently that they are improving their (notoriously slow) render times by moving to a virtual DOM model like React.
what they're actually saying is, React is getting even faster while other frameworks try are playing catch-up.
Angular has improved performance 30% from 1.2 to 1.3, and is slated to improve it another 30% from 1.3 to 1.4 (1.4 has a release candidate currently) - that is one of many reveals from ng-conf. Angular 2 has been revealed earlier that it is 5-10x as fast as Angular 1.x (depending on the task), which probably makes it notably faster than React.
Performance is still something that every framework is striving to optimize - this is only a good thing when teams announce that they are working to improve their performance, mobile is still hard with the DOM.
Each of the frameworks draw positive influences from each other, and don't hesitate to give each other credit - the React team praised the idea behind ng-animate for programmatic CSS transitions, the Angular team directly drew from React for the idea of unidirectional flow for Angular 2 and built on top of Ember's route-recognizer for the new router.
