The proof that they bring for this is that if one performs a bris later on, he is no longer chayav kares. Where do we find that one can be chayav misah or chayav kares but then rectify the situation by performing another action? Never. Once one is obligated, the obligation never comes off. This is proof that when one is chayav kares for not performing a bris milah (and for not bringing a korban Pesach), it is not a death sentence. That is why it can be removed later on. Since it is not a death sentence, we cannot apply the halacha of kim lei bi’derabba minei.