I could be wrong here, but I'm 90% sure this is what a friend of mine refers to as "semi-official" artwork; i.e., it was drawn by the creator/main character designer of the show, but, for obvious reasons, couldn't actually be used in any official promotions. Also more or less puts Diakko into same status as Nanoha x Fate; aka "officially, we're not allowed to say it's canon, but it's totally canon"