Why does the poet use aʿnī in “thumma Ḥujran aʿnī ibn Umm Qaṭām”?

al-Ḥārith ibn Ḥillizah: Ḥujr, Royal Kinship and the Dhū al-Majāz Covenant (verses 56–67)

To identify which Ḥujr is meant
To date the battle
To command the addressee
To deny his lineage