Insurance companies are in the business of making money by denying /minimizing claims, and not in the business of compensating customers for their losses.

If you use a discount code for which you do not qualify and have an accident with a serious damage cost, you provide your insurance company (doesn't matter, the rental company, or your credit card) an easy excuse to deny your claim.

If you don't have an accident, most likely no one will care about the code you use