Luffy and zoro arent the only ones capable of beating or hurting flame off lunarians
Do you genuinely believe current sanji cant defeat a lunarian?
Or believe zoro couldn't beat a lunarian before he unlocked conquerors?
Zoro and Luffy probably aren't the only people that can hurt them in flame-off mode, but they're the only ones that we know of that can defeat them within whatever time limit they get b/w flame-off and flame-on mode. Flame-off isn't even an easily exploitable weakness according to this chapter.
So even if Lucci and Kaku have strong enough attacks to hurt the Seraphim in flame-off mode, that still doesn't prove they can defeat them before they go back to flame on again is what I am saying. I genuinely don't have the language skills to make this simpler than that to understand.
As for Sanji, I am willing to give him the benefit of doubt against the Seraphim as they're not fully developed, but not against King.