It's super easy, for example in naruto many weapons were used, for example kunais or swords. No one was a kunaiman or sasuke who used a sword was not considered a swordsman. Why? Because for them, the weapon was just one of many things in their repertoire to fight. They can use a weapon/sword but are not solely fighting with that. For example the same goes for king, he even said this himself to zoro.
But zoro pursues the way of rhe sword, he only uses swords to fight, because he IS a swordsman. I mean, zoro could also punch his opponent but doesn't. Why? Because his repertoire is (intendly) limited to swords, which makes him a swordsman (just like mihawk). As for shanks, we don't know but it's highly possible, that he doesn't only use his sword.
I don't even understand why this is so hard for many to understand lol.