A perfectly elastic collision would still be the same - the energy is still conserved and divided equally between the two cars.

Assuming the cars are of equal mass. If not, the heavier one will drive the other car backwards faster, and the energy will not be divided equally between them. But the sum will remain the same.

I’m pretty sure the basics were covered in AP Physics when I took it but have no idea what the curriculum was like when he was in high school. That said, I agree that it’s generally a smart ass answer but a) teenage boy and b) I would be incredibly unsurprised to learn that a child of academic economists who goes on to get a PhD might be the kind of insufferable high school student who reads ahead and makes sure everyone knows about it.