Typical Oda, he'll mention or show something years ago and you won't even give it much thought, and then years later that little piece of info will come into use again.
For example Haki was first shown right at the beginning with Luffy's backstory on how he ate the Devil Fruit, when Shanks used it on the sea monster, and then later seen used in the floating city ark, then again with Shanks when he meets Whitebeard, and I also believe when Zoro was fighting Number one and cut steel that was him using Haki.
