I'm not entirely convinced that this is true, either. At least not in some general, a priori, way.

Doing stuff "the right way" with React or any other framework probably still involves a fair amount of ceremony, boilerplate, and testing. Would your hypothetical React dev be faster with React than some other framework? Probably--if that other framework is similar in scope and complexity. But, otherwise, I just wouldn't assume that an expert in FrameworkX will always do better using FrameworkX without regard to the specifics of the project.