"Take joy! I am your guarantor for getting 500 dove-footed nymphs."

The Nanda sutta seems contrary to what the Buddha teaches with regard to sensual desire. Therefore, I don't understand how this was a wise approach in the case of Nanda even though Nanda acquired arahantship afterwards. Perhaps I'm missing something obvious (it's still morning here!) but how did the Buddha's promise become a catalyst for Nanda's ultimate realization?