He is more than just a swodsman.
It's fine as long as his swordsmanship is inferior to zoro's.
If it came to king using anything other than swordsmanship, zoro isn't obligated to limit himself to swordsmanship too.
Being the strongest swordsman, means to be better than enyone in a sword fight.
As long as it is not a competition like luffy vs zoro, where they compete over the strongest fighting style, zoro is allowed use any other means to fight.
Zoro vs king is not a competition, it's a war, the only goal is to win.