This is a sally, and that's exactly how it worked in RTW as well. The besiegers didn't attack him, he attacked them with the garrison. If they had attacked with a spy in the settlement, he wouldn't have had the "Start Battle" button at the top; the battle would have just begun and he would not have had the opportunity to redeploy troops. If they assaulted and the timer ran out, it would have been an automatic victory for the defenders. The only time you ever see a draw is when you sally out and the besiegers are still there when you conclude the battle.