Omni sans is well omnipotent and lb sans ain't so that slight difference makes it super easy for omni sans to win, Thats why I think he was holding back.
I JUST REALIZED LAST BREATH WAS DISTRACTING OMNIPOTENT SANS WHILE THE OTHER SANS GETS THE BOOK OF EYES AKA WHAT OMNIPOTENT SANS IS PROTECTING AND A BOOK CAPABLE OF TRAVELLING TO OTHER REALITYS