I understand that the jaw physically "locking" is myth but would you agree they are more likely to bite with the intention of not letting go as opposed to biting and retracting? The pitbull that ripped my grandparents neighbors collies leg off would not let go. Not even when being hit with an iron bar.
My maltese cross who is 4.5kg has the ability to grab hold and hold on for dear life (when playing with a tug toy) - so any dog can certainly hold it's jaw tightly clenched - while 'lock jaw' might be a myth, a 20kg holding on to the death is certainly not.