We know Who's Who wasn't imprisoned in Impel Down, because prior to Luffy's invasion, only Shiki ever escaped from there.
Well, not exactly, because Morley escaped as well. And considering the "demoned away" thing that turned out to be the people from Level 5.5, there might've been other escapees that the guards brushed off as such. Considering the breakout two years ago, I would rather argue that Who's Who wouldn't have much of a grudge towards Luffy if he were the one responsible to get him out of Impel Down? Plus everyone came to Marineford, and there he would've been exposed to many people who should know what he looks like.
So if Oda doesn't want to entangle himself in a net of plotholes, Who's Who definitely can't have been in Impel Down at the same time as Jinbe, meaning he saw him in another prison. But Mariejois would mean that Jinbe was there as well, which we know nothing about. And there is only a tiny gap in the timeline where Jinbe could have been a slave in Mariejois - Who's Who was imprisoned 12 years ago, Jinbe became a Warlord 11 years ago.
But now that I think about it, the gathering of the Warlords before the war was in Mariejois, so it might've been because of something like this that Jinbe was there... this is a lot to think about. :yasu: