Something's definitely weird... unless it was, in fact, a one-time use code and someone else already used it? I hadn't tried before, because none of the books on the list particularly interested me. So I just posted it in case it helped anyone else. After reading your comments, I just logged into Kobo and tried the code for two different books on the list and it didn't work for either.
So either someone enjoyed it (I hope so!) or Kobo is even more screwed up than usual...