It doesn't matter who and who not cares about the WSS title. You automatically become a contender for the WSS title or even become the WSS yourself, if you display the strength necessary. Whitebeard didn't care about being called the WSM either, yet he was given that title though his actions and power. Mihawk himself isn't even interested in being the WSS; he waits for the swordsman to surpass him, even trains him.
Haki is an essential skill for a master swordsman. Mihawk said one has to achieve hardening, for every scratch on one's blade is a shame. Hyo said that through using Ryuo, the swordsman becomes one with his blade. And not long ago we learned that you can infuse your swordsmanship with CoC too.
Problem is; Whitebeard also had top level Hakis, he had a supreme grade blade too, he even had the world's strongest paramecia on top of it. While Roger was just a swordsman. Which shows that the skill itself (way of the sword, or grace as Mihawk phrased it, which makes a sword powerful) is immensely powerful. You can't argue with Haki since all the top tiers have high level Haki and Roger in particular incorporates it with swordsmanship.