I think the culprits are various or even multiple.I can see various root causes of stuttering on this sub and my stutter was caused by environment(it's gonna be intelligible if you really want me to explain),making sure that I was not a PWS until 12 with my parents and I was completely healthy in speeches. BTW one thing we should keep in mind,that if random psychological therapies are not effective,it won't mean stutter is never psychogenic,maybe just because therapies themselve are not effective on psychological issues cuz psychological issues are not that easy to solve,like depression,anxiety disorder,social phobia and etc. are still a hard problem for ppl to work out,not that easily. Good news came that one kind of meditation called Ecopipam is on clinic trial already for approxiately one year by now and the recuit will be finished at the end of this year,and the trial is very rigorous and relible: https://clinicaltrials.gov/ct2/show/NCT04492956