Rather than parrying the thrust in Tertia, you can counter it before it even begins. As he is gaining your sword, our “strong constraint”, disengage under his blade to the inside. Then immediately gain his blade in Quarta and thrust as per illustration 8.
Care must be take to not let your opponent complete his constraint. L’Ange hasn’t spoken of this yet (disengages are formally covered in chapter 10), but we know from other masters that if you disengage too late you’ll be struck during said action.