I believe you’re correct! It’s largely because of the anticipation anxiety of getting the proper words out when talking to someone. My stuttering is worse when I have things already pre-loaded in my mind that I’m about to say. I talk to myself out loud as well and I too have none or very little stutter doing it. But I think it’s because we’re going off of a stream of consciousness. If you just let words slide out and skip having to process what’s being said, it bypasses the stutter.