I know it feels odd to call it a 'draw' when you feel you are winning, but the win conditions are not do 'better' than the enemy, they are, "clear all enemy units within a five minute time limit." If you can't do that, you've failed. They don't call it a 'defeat' because they reserve that for the special case of where you've done so bad that you've been completely cleared by the enemy in five minutes.
But it is a defeat. One of the win conditions is to do it within the time limit. You didn't. You don't need to be rewarded for that.