I agree with Jiro that the typical mind fallacy is likely a large factor here, but I only see it as affecting the quantity/sophistication of worlds-inside-head, and not the quality that most people have them (I won't go as far as the quote's claim that all people have them, though.). I still agree with the sentiment that it's important to remember that people's inner lives are often much more complex and subjectively rational than we may see from the outside.