The Amex Platinum code is a discount code or CDP in Hertz parlance--not a promo code. Some CDPs are available for anyone to use, while others require you to be an employee of a certain company, a member or a particular organization, etc. Promo codes are a bit different, but even there, some of them are tied to specific organizations, such as coupons that are for AAA members only.

The bottom line here is that you should use codes you know you're eligible for to be on the safe side.