Yes, it's obvious that current Sanji is stronger than his siblings. But that's not the point. Since there was never a fight between him and King. All that happened was that King tried hard to breach Sanji's durability, who just tried to protect Momo. So all we are comparing is, how successful was King in hurting and damaging Sanji with a free try hard hit, vs how easily Katakuri trashed Ichiji who has superior durability due to the additional exoskeleton. Not to mention that Ichiji out of all siblings has incredible damage output with his Kizaru-like lasers, just putting holes in Oden and the other Big Mom pirates, with Oven lying on the floor like trash and bleeding. Yet Katakuri wasted him.