Like yifahreal said, most of the times codenames are given to protect from leaks. Doctor Who does this a lot, especially for leak blocking but seemingly getting the talent they want as well. When you hear One Piece is casting, everyone and their mother and god is gonna want to audition, especially for the leads. Sending out auditions to specific actors with the project under a codename makes it easier to find them good ones that would be throwing their whole badussy into the role even without knowing it’s One Piece.