It's more likely the seller bent the phone and put it in a case so you won't notice. And, Apple never officially was replacing bent phones. If you can plea your case and the genius guy is being nice, he might offer a replacement.
Honestly I don't think he did. He had it in the original box when I purchased it with the case off. Didn't seem like he was hiding anything. I put it on a flat surface (without a case on; screen face down) when I purchased it and didn't notice a wobble, yet now I do.
Ive had it in my front jean pocket and the jeans I wear are pretty loose so I don't know how it could have happened.