This is true. I unfortunately had to use my Platinum insurance last year and Amex insurance confirmed that all costs paid with any of your personal Amex cards would be covered. Only expenses paid with your Amex card will be covered unless the merchant did not accept Amex (in which case you can pay with any other card). The insurance even covered our part of a family safari trip that one family member had initially paid on behalf of all travelers (i.e. not paid with our Amex).