I would refute his claim, tell him a former 911 driver would confirm he could see this easily being caused by a 911 impacting a light bumper on the rear of your Seat, I'm sure Gibbo, Mark and others who have/had them would be of a similar view. It is a stupid claim that it could not have been caused because there is no damage on the 911 or because his numberplate is intact. The realty is if the paint hand't flaked when you pulled the dent out there would be no mark on the bumper of yours either.
Trouble is without proof it's difficult to win, though I would suggest if she confirmed this at the scene you should write back and remind him of the fact and tell him it is being passed to the legal team of your insurer who will take it from there.....assuming thats the route you took.