It's "Swordsmanship surpasses Shanks." as per Eten's post. Which is a lot more vague.
And because Oda likes to keep things vague.
Despite it being crystal clear, he has never once in his life made a clear statement of Zoro > Sanji in manga. Only Vivre Cards, SBS, etc etc. Mihawk is outright stated to be stronger in VC, too.
He likes to stir shit some, what else is new? Unless you think Zoro will turn off CoO, CoA, CoC, strength, speed, etc just to have a "skill" battle for WSS.