I don't think the out loud or someone listening / reacting matters at all here. Suspect it's entirely this:

  >The thought that was comfortable as a vague impression has to become a sentence, and sentences have structure.

I often construct full sentences in my head. And have conversations with my mental model of some other person. In full sentences

I think there are tradeoffs though, and this has been a thorn in my side during technical interviews where you are expected to think out loud because:

1. Sometimes you have a vague sense of the shape of the solution, and ime it can be helpful to sit with it for a while before trying to shape it into words.

2. Talking out loud forces structure but it also rate-limits how quickly you can iterate through ideas to find one that plausibly solves the problem at hand