Sanji clashed to protect momo, he didn't try to kick King in a weakpoint to try to kill him.
King is a speedy Yonko Commander, CoO or not 1 moment will make a difference, yes Sanji distracted himself and it's his own fault from a combat perspective- his priority was Momo, but that still means it is a cheap shot.
Their transformations are incredibly fast: see Queen outpacing a base rocket with his neck (encumbered by Zoro but still fast).
Just take your loss, you bet Sanji would be as damaged as he was versus Doflamingo (slashed, bleeding and requiring saving) and he isn't and he didn't need to be saved by Zoro and Marco as you predicted. He is as damaged as Luffy and Zoro are currently, minor scuffs/bruising no bandages required.