Have we seen anyone other than Emma, Regina and Henry interact with the writer? Hmmm...
Again, I've been assuming and discussing for awhile that he's the writer of the Once Upon a Time book - someone had to write it, and since it knows everything about the curse, it's not likely to be Regina or Rumps, the only two that could. So he's of the town, but not - he must be some part of the curse itself. Perhaps he didn't exist in Fairy Tale land, but either his existence is a direct result of the curse or he was provided the information in our world somehow by the curse. Let's face it, without the book Henry wouldn't know what was going on, wouldn't have gone to get Emma, and the prophecy of Emma being able to break the curse couldn't come true.
Or perhaps while we might be thinking of the curse originally belonging to Rumple, he actually got it from someone else...like this guy. Hmmm, so many possibilities...