I know where you’re coming from. And I agree, stuttering is almost exclusively caused by the fear/ anticipation of it. However, it seems a lot of stutterers benefit from these kinds of breathing exercises. It’s known that a lot of stutterers do have a lack of neural connection to the diaphragm and since it’s an important part of the whole speech apparatus, it makes sense that it helps. I don’t say it eliminates it, but it surely does help. Also because of positive side effects such as a calm, meditative state when you focus on your breathing and a better oxygen-carbon-relation in the blood, which leads to lower cortisol levels > less stressed.