That guy is not a leading expert on stuttering and has view on stuttering is not backed up by science. I stutter when I talk to myself in an empty room and know of others who do too. That alone doesn't say much. Talking to yourself is not communication. Stuttering is more a communication disorder than a speaking disorder. Talking to yourself is also a very low stress activity that doesn't let many stutterers hit the stuttering treshold in their brains.