King has an akuma no mi, lunar powers, he flies with wings, throws fire, punches and kicks in most attacks, hits with wings, of course he is not a swordsman. Shanks only has his sword skills and haki, HE IS A SWORDSMAN, damn it, in this chapter he has used his first named attack and it is a damn sword attack.
The argument that being king of the pirates is more difficult than being the most powerful swordsman in the world is also not valid.
Because to be the king of the pirates you need people, a navigator, someone who knows how to read the poneglyhps, a strong crew, etc.