I don't understand your point? If it was a rental car it would be no different. Why would any company that is allowing you to use their loaner car cover damages that were caused when in your possession?
Now as far as the insurance issue...this is one of the most frustrating parts of my business. So many companies have started doing this. But did you call the police, or have any other witnesses? The attorney will be a waste of time, you can file with your carrier and pay your deductible and they will try to go after the other company (subrogation) it many times becomes much to costly to litigate so they don't recoup any money on these small claims...that means you don't get your money back. These direct carriers namely Geico has started this, now progressive and nationwide all seem to do this everytime if their insured hits another person and it is a word vs word situation. Even when it is clearly the other drivers fault...it is just impossible to prove and the other companies know it. Next time take pictures, get names of others, call the police. Now personally since the police don't do police reports unless there is an injury, for me I may or may not call the police.
What was the reason the other carrier denied the claim? I wouldnt give up so easy...many times that is what they want...they deny...you go away. Call and after the first person tells you it is denied, ask for a supervisor and keep going you never know they may assume liability. Best of luck...your issue is with the insurance company...actually the dishonest person that hit you...not Lexus.