Base King is basically Sanji on steroids.
- harder body
- stronger fire
- hits harder
Also wielding a sword for lethality, maybe even having CoC.
And this combined with a flying dinosaur fruit.
I can understand why
@Chrono thought King will face Sanji, for all of the similarities they have. He just misinterpreted King's purpose. Not being similar to Sanji but a stronger version with more abilities and a dinosaur fruit on top. And a stronger version of Sanji can only be beaten by Zoro.