The Strawhats invent the codename on the assumption it’s a joke / fairy-tale flourish; the audience reads it the same way. The narrative-mechanic is dramatic-irony name-etymology — Sanji is a prince (born to Germa 66, the assassin-prince military-family). The chapter cliffhanger pairing (Sanji + Vivi as prince + princess) is the first on-page Sanji-with-princess-coding wardrobe-and-role frame.