On a serious note, it's true that everyone who uses a sword in battle effectively, is a swordsman. King doesn't call himself swordsman because he doesn't want to identify with a particular style, but swordsmanship is part of his arsenal.
That being said, we can however argue, which swordsmen the WSS title encompasses. And I would argue, not all of them. In the One Piece world, 99% of the people use weapons, because a weapon is in most cases a powerup, as it makes you stronger than with bare fists. A sword is often simply a weapon of choice. Big Mom's signature move, Spear of Elbaf, was performed with an axe too. Hakai was even used with a club. Whitebeard's weapon of choice happened to be a Bisento.
If swordsmanship just adds up to your versatility and is just a side power, I would not consider the WSS title to encompass him. People I think of are Fujitora, Kizaru and Big Mom. The sword is just their side power. Yes, Fujitora was called a swordsman, and he surely has great skill, but his gravity fruit boosts his power far beyond that. Same with Kizaru and Big Mom, who is a natural born destroyer who punches, uses Homies and creates storms. She could use an axe and would be just as effective.
But if swordsmanship is your main and most powerful asset, you are encompassed by the WSS title. It doesn't matter whether Roger used a gun, because his swordsmanship, which can also be enhanced by CoA and CoC, far outclasses a gun. Haki is just supplementary.