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Unfortunately they are probably right. A hard hit will often crack the AC condenser even if it isn't directly contacted. You will almost never see the leak- it leaks out gas little by little and you never see it.
I would ask to see the damage, just to confirm, but you will likely see either a seam on the condenser covered in oily dust film, or if it actually contacted the condenser you will see the damage. Either way, they are probably correct. We see vehicles all of the time with tiny pin holes in the condenser from rocks popping up on the road. All it takes is a tiny hit and it will make a leak and the driver has no idea.
Please let me know if you need additional info.