Well, most likely (let's say 99%) he didn't eat a devil fruit during his rivalry with Mihawk because for a swordsman is just an hindrance. After he lost his arm and was no longer a contender his attention switched to CoC, and we know he has one of the strongest, if not the strongest, in the verse. At this point he could have got one but maybe old habits still stick or there is something he knows about df that we still don't know. We have even his models/mentors that are top dogs but without a df (Roger, Rayleigh, but even Crocus). Or simply he considers them useless compared to what he can do with his haki (which has to be trained), maybe df could even be an obstacle to haki? Who knows.
But there is even a small chance he has one (or most likely one of his crewmates) since a lot of people was surprised to see him at Marineford so short after making Kaido turn tail. So the teleport like df theory has at least a tiny bit of ground (very small but still).
