Kid didn't defeat a Big Mom commander he only hurt him, he says so himself he just wanted to pick up a few chosen ones and now we know it was a poneglyph copy.
It was Urouge who beat Snack If Kid would have beaten a commander, Oda would have said it very clearly like Urouge I think. And then whether Kid beat a commander or not, he doesn't care he succeeded in his objective in any case.