-는다더니 quotes what was claimed and then shows what actually happened. Sometimes the claim collapses: 안 먹는다더니 잘 먹는 형 'he said he wasn't eating, and there he is tucking in.' Sometimes an old saying is confirmed instead: 사랑하면 닮는다더니... 'they say couples grow alike — and sure enough.' Nearly every line here is an on-screen caption pointing at the mismatch, which is exactly what it is for.