Same thing happened to my mom's car, in the T&T parking lot no less lol. She was reversing from her stall, probably came out a few inches when she saw another car reversing as well. My mom had come to a complete stop and the other car hit her rear. They admitted fault and we went to our broker to deal with it. After a few days, the other driver just straight up ignored our calls/text/emails. Our broker said that it's a 50/50 and I'm guessing the other person knew that after talking to their broker.
In the end, we would have gotten dinged on our insurance if we claimed anything. We decided to get a new bumper instead.
It's super shitty when this happens, especially when the other driver knows they are at fault.
PS. We had a witness as well. Our broker pretty much said that a witness is useless in this case.
PSS. The only time you can fight it is if your car was still in the parked spot and you never moved it.
Four wheels move the body, two wheels move the soul.