King seem to be attacking from his front though he could see King's attack, we have yet to see who's who fighting style it's obvious he isn't the same caliber swordsman as King. Franky and Sasaki have been fighting for a while i don't think we have seen him pull out his sword yet but at least Franky's shogun can counter it with his own sword.
Firstly we don't actually know how skilled King is, remember skill and strength are not the same, but even if who's who or Sasaki are not the same caliber sword users, then we must also remember that Jinbei and Franky and not the same caliber combatants as Sanji, nor do they have the same caliber tool designed to take all types of damage, from fire, electricity, bullets and sword attacks. Also slicing through Marco is something that who's whos and Sasaki can replicate, in fact, anyone can cut through Marco, because he isn't made of a solid material nor is he dense it's just a Flame. Take a knife and try cutting through a rock or a piece of metal, that will not only be harder but actually effect your tool, that is basically what you are doing when trying to cut the raid suit. And if Izo can disarm King in the way he did I have a good feeling Sanji can as well in the heat of battle or maybe even catching him off guard in his entrance. Ultimately what I am trying to prove to you is that Sanji can fight King and the sword excuse does not hold water when met with the other examples; Who's who, Sasaki, Cracker, Smoothie, Katakuri, Jack, Queen, Big mom and King ALL use swords or in Kata's case a trident, there is nothing special here whatsoever.