Well, you still have to _perform_, so whether or not you want to call it "all mental", the end result is what matters - and if you stutter in performing, well, then that's where the stutter has shown itself, in the act of speaking. But sure, part of stuttering, I believe, is actually this over-thinking, because we're well aware of our situation. I just don't see why we should reduce it to that (and I don't think there's any science to back that up either).