Just had this exact same scenario play out for me today on the way out of town for work.
Same quote from the dealership and everything.
I am superconcious on the interstate about rocks and always try to keep a considerable distance. I guess rock damage is inevitable. Throughout the day, I got on my hands and knees and inspected the radiator and never saw anything. Just got the call from the dealership and they said to come take a look at the damage. Ugh.