It concedes a point and then says it makes no difference: however much you try to keep it secret, it will not stay secret; low-calorie or not, if you eat that much; even if he really can fight, why put up a sign about it. The frozen form 뭔들 means whatever it is, which is the compliment in the line about someone looking good in anything. It sounds bookish, and everyday speech uses 아도 and 어도 instead. Three lines here are false matches where the letters are just the plural marker on a noun.