I think your guess is right. 덕 in 덕질 stands for "Otaku", a Japanese word meaning people who obsessively dig something, or a maniac. 질 is a suffix meaning an act or practice in somewhat derogatory sense. So 덕질하다 means to indulge in something maniacally, especially enthusing over certain entertainers or artists. It's an obscure slang word not even in the dictionary. Most native speakers wouldn't even understand it, let alone use it.