It's interesting to me that this person completely lost their internal dialogue. So many people strive to do just that via meditation etc. I wonder if it's different, in the same way that people crave silence but when they are in an anechoic chamber they freak out.
Our internal dialog acts as a reference point for our existence. Just ask Decarte.
There's also an interesting physiological component to this that I just found out about recently. I have an Internet friend who is recovering from laryngeal problems related to their chronic disability. At their weakest, they said they had no inner monologue or that it felt "raspy" at times. Turns out there are very slight movements in the tongue and throat muscles used for speaking when you're "talking" in your brain.