Ben fights primarily with a gun and is a more
powerful marksman than Yasopp. I see where you are getting at. Maybe Yasopp is just more proficient with a rifle, has greater skill, can aim shots Ben can't, while Ben is more about raw power (or maybe not, maybe Ben has the best skill too), but to be the strongest swordsman is not only about skill, but about power and Haki too. That's why Kuina complained that she will never be able to be the greatest, just by virtue of lacking the physical strength.
If Shanks could shatter Zoro using shockwaves, do you think Zoro would consider himself the greatest swordsman?