i just realized something:
the pirate king is not just luffy, it's the monster trio.
luffy: body
zoro and sanji: wings
the wing's analogy means that luffy, zoro and sanji are parts of the same entity, i'm not sure about the rest of the crew tho.
usopp said that luffy can't become the pirate king on his own
robin emphasizes on the importance of the wings in the journey to the pirate king
zoro and sanji acknowledging that winning their respective fights is what's gonna make luffy becoming the pirate king come into sight, they don't know about the defeat of the flying 6 (at least who's who/black maria/sasaki)
luffy himself declared that he values sanji's help to the point where he can't become pirate king without sanji
it's not mainly related to his cooking skills, but also his strength
sanji was having serious issues adjusting to the required powerscalling of the new world, this recent development fits perfectly with his mission and status, and it's a long overdue because he should've already unlocked his true potential even before his introduction or at least during the timeskip, oda delaying it makes no sense imo.