The river Sajo, is a minor one, not much of a width/depth even flooding. Full cavalry, with no army train living on the countryside makes logistics much easier. Most of the Mongols' major river crossing was fording or even swimming - no problem for cavalry without any significant train. Or when the rivers were frozen. Granted, crossing a major river with significant opposition on the other side, either by fording or swimming is very difficult. As the Mongol mobility relied on foraging, withdrawing to walled cities with all the food available and scorching the country would have denied them that mobility advantage.