Well, one problem is that people (or is it only in Poland?) have this common idea about folk that it's only merry, which, of course, isn't true. But still, DCD are inspired in sacral music as well, not only secular (and folk is secular).
The term Modern Folk, if we'd like to be anal, suggests something that is reflecting the modernity and comes from the "lower" classes, so, in my opinion, Hip Hop would also fit there - unless we accept that Folk can exist only in rural areas.
Of course, there's this branch of music called "Neofolk" or "Dark Folk, or "Apocalyptic Folk" - nice bands like Current93 and Death in June - and their music is not very different in atmosphere to DCD. I'd certainly be less surprised to find DCD in "Neofolk" than "Electronica" - especially considering the fact, that DCD on most of their albums use acoustic instruments!