The Jin also make the decision to stay on their horses, with none of their cavalry dismounting. In the ensuing chaos, true tactics are nearly impossible, as both sides have severely hindered movement from the terrain. Your archers struggle to guide their horses into strategic positions to launch volleys of arrows, and eventually the Jin emerge victorious in a slog of a battle.
You chose a non-historical decision! In real life, Genghis was quite willing to make adjustments and use creative tactics to win battles. He did not rely on one single advantage to create his empire. Read more about how your decisions differed here.