The previous image, khurbat al-mazād, showed water rushing through an opening. The next image, ḥazm Thahlān, gives the movement a particular mountainous direction. Both support the same martial claim from different angles: one shows force; the other supplies route and wounds.
This is progressive visualisation. The poet first suggests how retreat looked, then where it led. Reducing the verses to a single sentence, “we defeated them,” loses the movement of the poetry.