It's in some specific terms and conditions for various AmEx cards. I looked up the card terms and conditions that appear to be available here:
Car Rental Loss and Damage Insurance - Terms | American Express.
Ours is the Gold Delta SkyMiles card which on this website includes the following exclusions:
Car Rental Loss and Damage Insurance does not cover rentals of :
1. expensive cars, which means cars with an original manufacturer's suggested retail price of $50,000 or more whennew;
2. exotic cars regardless of year or value, including but not limited to Chevrolet Corvette, Toyota Supra, Mazda RX-7,Dodge Viper and Stealth, Plymouth Prowler, Mitsubishi 3000 GT, Nissan 300 ZX, Jaguar XJS, Acura NSX,Mercedes SL, SLK, S Coupe and E320 Coupe and Convertible, BMW M3, Z3 and 8 Series, Cadillac Allante andall Porsche, Ferrari, Lamborghini, Maserati, Aston Martin, Lotus, Bugatti, Vector, Shelby Cobra, Bentley, RollsRoyce;
3. trucks, pick-ups, cargo vans, custom vans;
4. full-sized vans, including but not limited to, Ford Econoline or Club Wagon, Chevy Van or Sportvan, GMCVandura and Rally, Dodge Ram Vans and Ram Wagon; . . . .
We only recently signed up so I haven't received my actual policy in the mail and it might be different. Fingers crossed I have the more limited exclusion you cited! My main caution was to make sure whatever you rent is covered by the specific policy that you are sold.