The other way of looking at the weakness, that i am insinuating at, is from your perspective, you who won the fight, and yet would say:

I thought he was weak. But he made me realize that the weakest was myself.

That looks an admission of regret, namely that despite wining the bout, you hadn't really found the satisfaction expected from winning. Your final mindset is overall that despite wining, you were actually the weaker.

Walking away, despite his (the loser) persistence to keep the fight on, especially since you knew/saw you were obviously wining the fight, would have , in my opinion been the highest demonstration of strength.

There is probably another angle, but i think this is worth considering. Anyways, that is the beauty of the narrative - leaving many angles!