Funny thing, they were all dead serious but anyways it doesn't change anything, the moment King had a meitou and Queen triggered by everything and also sending the t6 after Sanji, the matchs-up were settled. Oda is smooth, he loves to give small hints and if you are smart, you should have catch it.
But i guess you absolutely want King vs Sanji because their clash was too one sided and Sanji couldn't do anything to King.
Oda used him to save Momo and gave him a role in the war, you should be happy with that!
Imagine now if Zoro fights King, the salt is gonna be tasty