I think Dan is making a different point. He's saying functional components are just like regular components in terms of speed. The author of this article is saying use functional components so it's clearer when a component needs to be split up.
It doesn't help that several popular React & React Native tutorials out there, for understandable simplicity, opt to use "renderSubThing" methods. In the examples and in my own code so far, I mostly find those sub-render methods to be a proxy for "I don't want to break stride to make a new component, so I'll jam this stuff in a method and refactor later." That's great, so long as the refactor actually happens.
(Hm, that'd be a great candidate for a vim refactoring plugin...)