Agreed. However, the OP said his “fluency increased greatly.” So like I said, as long as he’s not being hurt or taken advantage of why should he stop? Even if the stimulation is simply heightening his awareness of these areas (i.e. throat, palate, tongue) giving him better control for speech, maybe that’s helping. Even if it not “strengthening” them, it could be doing something else that we’re not aware of.