If Tidas hadn't of caught it, then Marco could've demanded more people from the Highlands if the threat of war was eminent. Or if Alcon loss a massive amount of mages in a battle, Marco could refer to it as a dire situation, and indoctrinate twice the number of mages agreed to, or possibly more.
Not being a fool, Tidas knew that his brother had written it in such a way as to leave the treaty open to 'interpretation'. Meaning that during Marco's rein: he could indoctrinate every mage within the totality of Alcon, which technically included the Highlands. So long as he had another law or order dictating the requirement of it.