You shouldn't listen to 'Someone'!
For the purposes of criminal law, private property is (in many, but not all cases) regarded as being part of the public highway if the public have free access to it. For example, you can be prosecuted for 'driving without due care and attention' or 'dangerous driving' on, say, an Asda car park. (However you can't be prosecuted for, say, not wearing a seat belt on that car park).
It would be a ludicrous situation if civil law applied different standards. For example, if a 'boy racer' slammed his car into yours while practising hand-brake turns on supermarket car park, he could be charged with 'dangerous driving'. It would be illogical for the insurers to treat the incident on a 'knock for knock' basis. In all circumstances, it's the person who has acted negligently (or, ultimately, their insurers) who must meet a claim for damages.
Of course, with some minor incidents on private car parks, insurers might agree to settle on a 'knock for knock' basis simply because they know that one side will end up with massive costs if the case has to go to court. However, that's definitely not the same as stating that there's a rule that all incidents on private car parks will always be treated in that way.
Chris