Oda changes his story. It's not the first time and won't be the last.
Earlier in One Piece:
The DF user knows the name of their fruit when they eat it.
Luffy ate the gum gum fruit.
Calls his attacks gum gum attacks
Luffy telling us he's not a Zoan type.
Current:
Luffy has the Nika fruit and it's a mytical Zoan type
The gum gum fruit is not the real name. That just we call it.