First of all, how do you know it was a mistake? Maybe (as someone else pointed up thread) the had a certain number of sign up offers and they shut down the promotion when the quota was met. Second, if it was a mistake, I'm sure someone from AA would've said something already. I believe that they are going to honor it since it is not much different than a paid challenge except (and I know this is big) for including EXP. About a month ago I've got an email offering me Gold status if I flew 6,000 before the end of the year (no fee required). In that event, the terms and conditions explicitly said that it was targeted to the email recipient. This time around, no conditions were noted, except the one of not being an elite member of the program. Regardless of what we think, AA will do whatever they think is the best for them and move on.
Good luck to the one who registered!