One possible explanation: they had no or very few F customers and an overbooked Y cabin. The number of J/C revenue/award passengers was fewer or equal to the number of F seats open, so they moved all J/C passengers into F and filled J/C with overflow Ys. Trust you enjoyed the service.
BTW Can you tell us if they sent you to the BA Terraces Lounge at JNB or the SAA First Class Lounge?