Somewhat out of my league in this thread but,I think I am.one of these people. I do remember a time before I had an internal monologue , in fact I remember the day in elementary school when I learned, after having been explained to me by my teacher, that everyone else was "talking to themselves in their head". I think I spent the next month or so obsessing over this new found ability. But before that day I was perfectly capable of thought, and conversation, and writing. Even now I can "switch modes" and have coherent thoughts occur, with no labeling or accompanying narrative. I can distinctly identify concepts and transitions between them but there are no words involved until I open my mouth. So I dont know if it was just a hidden background process before that day. But it definitely "feels' different when its in the foreground or back, or nor there.