swordsmanship and overall power ain't same thing
Let me ask you, what changed between Zoro's Oni Giri in Chapter 1001 & Zoro's Oni Giri in Chapter 5 or 6 i think?
Dude, Swordsmanship is about becoming Physically Stronger, Faster, better CoA, better CoO, better Reflexes & Precision ... etc
Smoothie was literally trying to get bigger & bigger to gain more Physical Power just so she can throw a Flying Slash the size of Sunny.
Zoro was able to counter Fujitora's Gravity that literally creates deep holes but still fails at throwing a Flying Slash at Pica.
Mihawk literally cut an Ice Mountain that can cover an entire Island & Luffy from above of it can't even see the ground by just casually swinging his sword to cut a rookie kid. It wasn't even a flying slash, it was just an aftershock of his swing
Mihawk's CoA, Strength, Technique Speed, CoO ... etc are completely off the charts
All of these buff his Sword Technique & make him Greatest Swordsman, go re-read entire Manga