Well I'm happy for Sanji and his fans, but this happened way too fast with not enough buildup beforehand imo. Oda just conveniently gives it to Sanji in the exact fight he needs it, at the exact time when he needs adv CoC too, with exactly the right person to tell him how, without prior training, self discovery, experimentation or being on the edge of desperation. Turns out all Sanji was missing was Gaban 'telling him to be a king', lmfao Oda has really lost all nuance and proper setup and is just writing whatever as he goes along now. This is just lazy and I don't consider it good writing tbh.
Watch Oda do the same for Sabo and Coby when they need to face a god Knights in the final arc and Oda just give it to them on the spot because he only remembered in the moment they are needed. Proper build-up, character development, and foreshadowing be damned. Oda stopped trying after giving CoC and Adv CoO to Luffy.
My biggest issue is that Gaban's explanation is a lazy one, like 'be a king to get conquerors haki' TF is that even supposed to mean???