Jacob begins his exegesis of the future prophecy of Isaiah by dealing with the past - but a past that is known only prophetically. Because Isaiah's prophecies are explicit that the blessings stated will come through the Gentiles, it is imperative that Jacob lay the foundation for that necessity.
When the Lehites left Jerusalem it had not yet fallen, although the fall was imminent and the reason for Lehi's visionary command to depart. Nevertheless, it had been one of the Lamanite contentions that such was only visionary, and that it was not - the them - historical fact. Jacob reiterates the testimony of Lehi and Nephi with that of his own. Jacob has a personal revelatory knowledge that Jerusalem has fallen. Thus the glory of Jerusalem is no more, and the conditions that will require this restoration through the hands of the gentiles are now in place.