I agree with @asardiwal on the neuroplasticity aspect bc language development is very much in that category for sculpting how you pronounce and deliver words. Some may say it’s genetic some may say it’s acquired neurogenic stuttering(ANS) which is learned ie neuroplastic and there are example for all sides of the stuttering paradox and considering how all humans react differently to different stimuli there’s no saying that he’s wrong and your science is right bc the science on stuttering is still unclear there’s no certainty on why all that who stutter do. Quick example would be my sister and younger cousin who spends time around me (I stutter) after awhile they’ll notice that they’ll start stuttering I know they’re not mocking me or trying to stutter bc nobody on earth wants to stutter but it’s definitely something to do with the human psyche and dysfunctions of the hypothalamus and thalamocortical science has shown this possible and it’s feasible to understand why ie "you are your environment" on the other hand i could say it’s just a genetic thing since my grandfather stuttered one of my uncles stutter and his son stutters too but thing is I started stuttered due to a blood transfusion. So where does the underlying stuttering come from? Genes,brain development or alternative sources? Science does not know everything just yet stuttering is one of those things