I got this email too. Makes me feel a bit better about my chances of it staying in place while I'm waiting.
I had 3 friends try today and none of them qualified. Not sure if it's over or they just didn't meet the requirements. Maybe the purchased books in the past year or whatever was accurate. I know I bought one or two text books and a book for my mom this year.
I think one of the things it comes down to is if you have used an Audible free trial in the past, thus having already signed up. This promo seems to be focused on getting new Audible sign ups.