It is true that the 4,000 Zulus at Rorke’s Drift were a phalanx of the 20,000 Zulu army that defeated and killed 1,300 British troops at the Battle of Isandlwana in January, but I’ve never read anywhere that they retreated from lack of supplies.
The Battle of Isandlwana and the Battle of Rorke’s Drift occurred on consecutive days.