Hello,
I can make a suggestion in regard to how to manage the reservation next time this happens, only because I have been in the same situation. If! You don't mind if the first guest may not follow through, you can block the nights out, and when they are ready to book you can unblock them. This way you avoid the instant book slipping through. A word to the wise... I have had this happen to me three times in two years and the initial reservation only came through once and the other two did not. So....I won't be doing that any more! There is nothing wrong with just s simple...first come first serve attitude, you would save yourself a lot of hassle.
Airbnb is and shouldn't be responsible for the way you manage your reservations and I completely disagree with the previous comment, plead ignorance. Seriously, wrong.