Luffy in his words practically said that if it wasn't for Haki he would have lost his arm to Kaidou, after all Acoc and Acoa was what kept Kaidou "away" from him, without the haki imbued in his blow, Adcoc and Acoa, Luffy's finisher would be just another physical blow that wouldn't be able to bypass Kaidou's durability, have you stopped to think?
It was the Haki that made Luffy's attacks effective and not his DF.