Mind your steps, folks! There's a plot hole over there!
So, we all know that our favorite Marimo has no sense of direction whatsoever. But recently we discovered that he has a pretty bad memory, too. Why?
In chapter 462 Zoro knew that Ryuma was "The ancient "Legendary Samurai" who cut the dragon" (according to the translation). He even commented on the fact that, as a samurai, Ryuma must have a good blade. That means Zoro essentially knows that samurai are 1. swordsmen and 2. pretty GOOD swordsmen as they carry blades of high quality.
Thanks to whoever edited the article on Ryuma earlier I remembered that he learned this from Usopp, who in turn had learned it from Hogback. So obviously I was wrong assuming he knew of the samurai prior to the events on TB. But anyway...
In the latest chapter he had to ask uncle Brook what that "samurai thing" the guy on the denden mushi just mentioned is. I don't know how accurate the translation of this is but the vagueness of the phrase and the quotation marks make it seem like he never even heard of that phrase before.
That makes me think: What the hell? I know when he beat Ryuma, he said "let's forget this duel happened". But swordsmanship is this guy's life! How could he possibly forget such an essential piece of information?
Now I was chatting with KuroAshi98 and we came up with the following conclusion:
It isn't Zoro but an impersonator! Zoro was thrown off the ship while it was caught in the White Strom and replaced by Bon-chan who was also there, spinning in the current (as an after-effect of Ivankov's hormone treatment) after being thrown off Impel Down.
That explains everything! xD