You're definitely not wrong that that may contribute a lot - however, speech therapy is also a lot more effective for children, which would not apply to your reason. I think it's somewhere in the middle. I think that my biggest issue is that you're saying that NO ONE stutters completely mentally. There are people who go completely mute based on mental trauma, even though they're totally capable of talking. Stuttering is a lot less different from fluency in magnitude compared to something like that. Even if I'm completely wrong about the social aspect, that's definitely a counterexample to the purely mental claim