I believe that this is false. If you are a lord of a rural/woodsland/mountain/badlands region, I'm pretty sure you can still be made duke.
You can be named Duke of an existing Duchy; however, only Margraves can be named Dukes of a new Duchy.
That's what I meant.
Vacant dukeships are rather rare, and rulers often jump on the opportunity to name themselves dukes when the opportunity arises, due to the fact that 1) they can (wasn't always the case) and 2) it's the only way for them to be able to be lords. New dukes often result from the creation of new duchies, which as stated above, can only be made in cities or townslands.
