I suspect the author is exaggerating a bit when he says it's the only way he thinks. I also have the "narrative voice" but I absolutely can think about things without the voice, it just doesn't feel as much like "thinking", if that makes sense.

There are some things that are explicitly visualized, like an object I'm thinking about, where I wouldn't say in my head "I am imagining the sun. It is bright. Etc. etc.". I'd just picture the sun, and my "narrative" would probably be about why I was thinking about the sun, not describing it.

Same applies for more abstract concepts, just without the visualization.