أُولَئِكَ آبَائِي فَجِئْنِي بِمِثْلِهِمْ
Those are my forefathers; produce others like them if you can.
The poet uses «أُولَئِكَ» to highlight celebrated forebears and his own pride, then challenges the rival to produce comparable ancestors. The demonstrative does more than identify a group: it contributes to boasting, challenge, and indirect reproach.
The subsequent phrase «إذا جمعتنا يا جرير المجامع» establishes the setting of public rivalry. The speaker presents an acknowledged reality as though it were visibly before the opponent, underlining the opponent's failure to recognize it.