@sujinko If something happened in a kingdom to a number of other people between the king and the duke in the succession, a duke could end up becoming the acting ruler even if they're not actually crowned for whatever reason. That said it's also entirely possible someone used the wrong word at some point in the process of either writing this story or translating it because they didn't know duke territories can be called dukedoms? I can really only speculate but those are some possibilities for you offhand.
It's also worth noting that noble titles aren't just a one and done sort of deal. The Queen of England is also currently the Duke of York and some other stuff iirc? Different areas might refer to her as the locally relevant title more than as the queen, I suppose.