Here they say this because of the vocal cords, that it is possible to speak in a whisper without stuttering because of the slight pressure on the vocal cords... It seems to me that this is more likely due to the fact that for clear loud speech more jaw tension is required than for a whisper, therefore, the temporomandibular joint practically does not tense, therefore there is no reflex of restrictions for the movement of the jaw (stuttering), and, accordingly, for the muscles of the tongue, lips, etc.... I want to warn you, this is only a theory...