On the one hand, he's right: Writing helps refine your thoughts.
On the other hand, if your goal is to probe the validity of your thoughts, this is painfully inefficient. You'll get much further if you do one or two simple passes on your writing, and then pass what you've written around and ask for feedback.
I think I learned this in one of Haidt's books, and it has jived with my experience: If your brain has a bias or a blind spot, it's fairly unlikely you'll uncover it by pure thought alone. Perhaps if you put in as much effort as this author has, you'll uncover 20-50% more than the average person, which still leaves you with a lot of gaping holes. But outside feedback will uncover them very quickly!
I had a friend who thought like this person, and it was rarely hard to find flaws in his thoughts that he had not considered. He's as smart as I am, so it wasn't an intelligence flaw.