I believe the author talks about how he’s strayed away from proving this scientifically because it can’t just be proven like factor X lead to outcome Y. I think he’s just telling us what worked for him but he does claim to have a huge success rate amongst stutterers who take his advice/join his community. I wouldn’t have heard about it if a member hadn’t told me so I don’t think it’s a popular book just yet. so many people talk about failed speech therapy attempts so maybe it’s best to try a cheaper option first?