Later on the same video, Paul talks that he still stutter sometimes. But most of it is gone. Mushroom have the capability of changing one´s neurological pathway. To have more neuroplasticity. It does not cure stuttering or any other sort of trauma. It give´s you the chance to change your current pathway and limiting believes. But you can change or re-write them for worse... It just make your brain more plastic. Like when we were a child. If you experience a bad trip you can re-write a new worse pathway and maybe stutterer more than before. Or develop a new fear. So be careful. Especially with high doses like he did. He played a russian rulet if you ask me. There are plenty of people playing with drugs like mushroom and LSD that develop mental disorder or end up on the clinic. Personally, a low-moderate dose with a professional guided advice on a peaceful enviroment will increase my chances or success on changing my brain for good. That how I would go.