Truck in front in drive through rolled back into my car, Allstate says I’m 10% at fault because I didn’t honk?

A friend was driving my car, while he was sitting still the truck in front of him in a drive through rolled back (stick shift) and damaged the front bumper. Driver of the truck admitted fault, filed a claim with his insurance, who called the friend first then me. They got my friend to say he was looking down for his wallet when he was hit, so Allstate assigned 10% of the fault to him since he wasn’t paying attention to the guy in front of him and didn’t honk at him.

This seems pretty scammy, and because it is a “shared responsibility” claim they are making things take longer like dealing with body shops, rental cars, etc. They said there was no appeal process. Seems like if you hit a stationary car in a place they are supposed to be it should be 100% your fault.

Does it sound legit to assign 10% of the blame for not honking, and if not what are my options to get Allstate to change their assignment of fault?