Was this the Amex Premium Rental Car Protection?
This is the one you have to sign up for specifically and which incurs a $18 charge when you use the Amex Platinum card to pay for the rental. They stand as your first insurer instead of your personal coverage.
I had a bad accident (car damage only, no injuries) in France and invoked that coverage online. They took care of everything, all the paperwork and communication with Enterprise France and paid the 2000€ repair charge. It was the easiest insurance claim I've ever experienced.