Earlier quoted context omitted.
The existence of an "inner monologue" isn't really a falsifiable claim. Some people claim to have one while other people claim not to, but we can't test the truth of these claims.
In this particular case, is there any reason why we simply can't take their word for it? This is not a case of where if I say "weak" or "strong", most people pick strong because no one wants to be weak, even if the context is unknown (nuclear force for example).
As someone who was involved in spiritual practice of "stopping internal dialogue" for years, I can tell you that one learns that that dialogue (or monologue, pretty much the same thing) is quite subtle and complex, essentially multi-layered.
Typically, when you think that you "think about nothing at all" it's just the most surface layer that has stopped, and more subtle talking to yourself is still going on. It takes training just to become able to notice and recognize it.
After all, it's just such a constant and monotone hum at the back of one's mind, one learns to completely ignore it.
So no, I would not take a word of people who were not trained to notice their internal monologue that they haven't any :-)