"If writing down your ideas always makes them more precise and more complete, then no one who hasn't written about a topic has fully formed ideas about it." Apparently, even writing it down didn't help the author with this flawed deduction.
Whether logically rigorous or not, that excerpt seems to be the essay’s author’s way of rhetorically opening his reflections on the idea that writing verbally crystallizes thought.
As a reader, I do not believe that the author is making a claim that the quoted Paul Graham statement, reduced to symbolic logic, is in all respects valid or sound.