I thought it was a smart move on Jonathan's part, too. I think if the challenge had proceeded, Kalabaw would have ultimately lost, the challenge would have gone on for another hour or more, and they would have nothing more than the rice back at their camp they started with. His tribe ended up with better food and letters from home to boost their morale. Surely that was worth the cost of a little rice.
It wasn't a total loss for Tandang, either. They at least doubled their supply of rice and got to save their tribe the wear and tear of a more prolonged reward challenge. They might not have won the immunity challenge if they were spent from the previous challenge.