Just imagine: the king of the Okama Kingdom gets to know you, a guest, and teaches you about your own weakness, even though neither you, nor your adoptive father, nor your crewmates (especially your captain), nor even the sister you’re truly close to ever discussed it? Man, it’s bizarre. What does Ivankov actually know about Sanji? Just because he’s older or an experienced figure, does that mean he can read Sanji like a book, or something similar, and pinpoint his greatest weakness? And how much does that matter when we’re talking about Conqueror's Haki? After all, if unlocking that power depends on mindset, and Ivankov doesn't possess that power himself, what the hell is the point of his comment?