I think this post is simply a justification for being better at writing than speaking. Speaking is a clearer form of conveying ideas than writing. Speaking is also the most ancient & more evolved form of conveying ideas.
Another way to look at it is via personality traits. Good speakers are usually extroverts, hacker types are mostly introverts which is why they find it easy to communicate with themselves than others, let alone an audience. Which is why they are better at writing words or code but not that good at public speaking.
I think that there is no relation between being smart & being a better speaker, so it's wrong to call better speakers dumb. Its only a question of your personality type and that determines whether you will be a good speaker or bad one. Nothing to do with being smart.