I believe his assistants help him articulate his thoughts. The article implies that he writes everything meticulously himself, but that isn't true. As far as interviews go, interviewers usually send him the questions ahead of time.

Writing a book is not limited by typing speed, even with those kind of interfaces.

A book might be around 50,000 words. There are about 2,000 working hours in a year if you only work 9-5 M-F. Which means that writing a book a year requires only 25 words per hour, on average.