User blog comment:Mejiaj1/Which yonko is the strawhats and the heart pirates aiming for?/@comment-46.116.200.75-20120525131109

Luffy's reaction "oh... alright" are misleading. They give rise to the nagging suspicion that it ISN'T mum. There was a moment of recgonition on Luffy's part quickly followed by a decision.

Luffy might have been anticipating mum but was taken a bit aback, but not critically. Nami's reaction confirms thus much. Therefore it's easiest to assume it's KAIDO contrary to all reasoning and logic.