...Okay, so just because you have superpowers, doesn't mean you're a better swordsman, just saying. In terms of skill, CH has the collective skill at arms of every knight, every fencer, every samurai, every Roman gladiator and legionary, every Greek warrior and Viking lord, etc.--the entire collective of human knowledge regarding swordfighting, and how swordfighting works in real life. Just because CH can't swing around a sword the size of a skyscraper or go full shonen protag' isn't a detriment to his skill. In the case of the former, it actually makes CH better, in my eyes, because even in a world where magic exists, it'd make much more sense to apply that magic to a normally-sized, practical weapon.
..Not that I still necessarily agree with CH's ranking on the list. Swords, specifically, have only been around for a few thousand years in real life.
But I guess if you did the math of an average human lifespan, and how many people have lived and died since swords were invented, and how many of them learned how to properly use various swords, you'd probably still get a total experience level somewhere in the hundreds of thousands of years, at the minimum.