It doesn’t make sense to suggest that stuttering is caused by anxiety. Many people have anxiety; yet stuttering is a relatively rare disorder. If stuttering was caused by anxiety, the prevalence of stuttering in the general population would be a lot higher. And as for me personally, I stutter 100% of the time in all situations. Doesn’t matter whether I’m stressed or relaxed and confident, though my stuttering gets a lot worse when I’m sleep-deprived. So for me it’s definitely a physical/neurological issue.