We haven't seen "swordsmanship" from Roger. We have seen his weapon clashing with Whitebeard's weapon once. No swordspecific ability used. He used a named attack on Oden, yes, but that was rather a blast than a cut. Big Mom also used a named attack with her sword (spear of Elbaf) but the exact same named attack was used by a giant using an axe. So that named attack was not specific to swords.
That depends on what you see as swordsmanship. You are thinking of being trained in specific martial arts and techniques to further enhance your capabilities with a sword but that´s just that, technique.
People can have a difference in emphasizing different aspects of using a tool, strength, speed, technique and so forth.
Swordsmanship is simply using a sword to fight, it´s also the definition in pretty much every major dictionary.
Why do you think there are so many different weight classes in combat sports? Because a heavyweight can have the shittiest technique, footwork and everything else that is involved, simply crushes someone in Flyweight for example.