I don't think Oda's ever done a sleeper agent, why start now.
That's called not being repetitive. Any good writer shouldn't be repetitive.
And Oda isn't repetitive when it's about Sanji imo.
First, the Straw Hats usually only get one arc for their backstory, but Sanji got two.
Then he wasn't a punching bag back then, now he is a punching bag.
Then he wasn't a crying damsel back then, now he is a crying damsel.
Then he used to be Monster Trio back then, now he is not anymore.
Then he used to be a gentle man who treats lady with respect (covering Nami when her shirt was burnt in Little Garden), now he turns into a big pervert after spending 2 years in Okama Island (actually stripped Nami's clothes in Punk Hazard when he was in her body. A contrast to what happened in Little Garden).
Not impossible if he is the traitor now. He used to open the door in Ennies Lobby to help his crewmates, now he opens to the door to help the enemies.