What you just said is one of the reasons why Zoro probably wont fight King. The strawhats fight people who are best suited for their abilities. Zoro vs Pica had nothing to do with Pica being a strong swordsman......Zoro fought pica because pica's abilities were a task best suited for Swordsman. King is a task best suited for someone who can fly.
We have yet to see King fight with his sword so we don't have a full understanding of his swordsmanship. Will he have long range attacks like Zoro or will he be limited to just close range attacks like Sanji? Will he use his fire through his cutting attacks? No one can say.
The thing you must remember about Pica is thanks his ability to combine with stone it allows him to create golems, which increases his size, strength and power, and hide in his stone. His golem is what made him powerful and hard to beat. If his opponents can't get through his golem he doesn't need to worry about hiding or running away.
Since King is a swordsman in Wano and has been there for 20 years there's a very strong possibility he will have incredibly powerful/deadly ryuo/barrier haki. That power would make to too dangerous for Sanji to use his legs to block King's sword attacks. You focus too much on King's ability to fly instead of looking at his swordsmanship.
Then there is a strong possibility of King possessing Awakening. That would also make it very hard for Sanji to deal with. Even with the RS Sanji had problems dropping P1. Since Luffy encounter with P1 we now know that ancient zoan possesses incredible recovery power. Awakening enhances zoan's recovery power, making it even harder for them to be defeated.
Thanks to Zoro's powerful cutting attacks and strong swordsmanship he has a better chance at beating King and faster. Zoro wants to learn to cut fire, a power King possess.