In the sense of political power, they were certainly the equivalent of Dukes as they were the next level after King.
However, in the sense of land holdings, they are more like region Lords, or like BM Dukes were prior to the change, in that their geographical base was more or less fixed (at least the center was fixed).
However the new system allows for a wider interpretation, and now would game-mechanically allow for non-contiguous, non land-based duchies such as those of the HRE.
Maybe the year in Battlemasterra is really 1337? :-)