I'm pretty sure Genjutsu had something to do with it. Sasuke managed to hit Deidara with a Genjutsu from a friggin' mile away while he was flying. If Muu or Oonoki were flying beating them both would be as simple as forcing one of them to nuke the other with Jinton by making them think they were firing at Madara.
Though I'm pretty sure Muu would've avoided using it for that very reason, and tried his best to go hand to hand with him, or use jutsu that wouldn't destroy the whole area for fear of killing his subordinate. Taijutsu against the strongest Sharingan user? That's a loss 10 out of 10 times for anyone not graced with a perfect defense, super-speed technique, or S/T jutsu. He probably just countered everything they had until they conceded defeat and walked away, sulking.
In other words, it was a legitimate win most likely.