your first mistake was walking into a kia dealership
lol not to bash the thread but I have little respect for cars that just steal every possible idea from other companies and just sell cheaper lower quality versions
About your problem I don't think the dealership has a foot to stand on here, not only did they come to you days later they are trying to pin something on you that technically would have easily bin noticed at time of incident.
I would measure hight of curb and height of scratch on bumper, chances are they don't match. Secondly I would talk to the guy who did the test pilot with you and ask him why he didn't say anything when both of you were looking over the car after the test pilot. thirdly i'd ask to see recod of test pilots on that car, chances are there were several after you. Even if there was just one there is enough evidence in your favour for you not to be legally obligated to do anything.
If a dealership tried this with me there is no way in hell I would pay a cent.......and the fact you were in test driving a diff car the day before has no bearing on this incident I don't understand why they needed to bring it up