Chances are they're legit. I looked at your listing. I don't think some crazy person would book your stay when it's you AND your husband living there.
My first guest was having his home fumagated for termites AND he booked via some woman's account (who he said was his wife) and she was listed as living overseas. I thought for sure it was some sort of scam, but he turned out to be legit. I had him start up his own Airbnb account and rebook.
Nevertheless I would advise against accepting InstantBook in the future unless maybe you're trying to take advantage of being able to screen out people who have no reviews, which InstantBook lets you do. You'll get people who want to bring pets (against your rules), more people than allowed, no photo uploaded, 3rd party booking like I mentioned earlier, have bad reviews, etc, etc.