Zoro's ability to use the Nine Sword Style symbolizes his evolution as a swordsman.

At that point, Zoro will be at his strongest and this will allow him to bring out the best of his other skills as well, such as utilizing the King of Hell form, or, even utilizing the Ashura form.

especially someone that Zoro dreams to defeat, has access to all forms of Haki on a very high level, likely comparable to the likes of the Yonko. Clearly, any Yonko-level Haki user is above Zoro ...