In order to have comprehensive insurance on car you want to test-drive, assuming you have comprehensive insurance on your own car, you need to do a temporary transfer of your insurance for the "couple of days".
AFAIK, your car is covered on the garage's dealership / motor-trade insurance against theft, accidental damage or fire, while it remains in their custody, so to speak.
So subject to correction, I believe the insurance company and the salesman are right.