That in itself is a bullshit line of reasoning and easily showing ability to read selectively.
Zoro has not just been facing 2nd class enemies, he has been destroying them.
The only exception to that was Daz Bones and in that Saga Luffy defeated Crocodile completely with help of plot. 2nd arc Skypea, where Enel couldn't even damage Luffy with his main power mean while Zoro mid diffed Enel's strongest subordinate.
Yeah Zoro's fighting potential has been some what wasted because Pica was a fodder to him, so was Killer, Apoo and Hawkins. Kaku and Ryuma were okay opponets but were still no where near strong enough to push him to brink of his fighting capacity.
You have to compare times when Zoro has been given a chance to fight strongest adversary with Luffy such as Ohm and Pacifistas in Sabaody and Zoro did just as good as Luffy in those fights, if not better.
His fight against two Yonko is also a similer scenario as Oars scenario and we will see how this plays out.