I'm no lawyer but I didn't see anything in the Amex link which addresses liability -- only physical damage which the OP already has. Don't know if the laws in Mexico have changed but in the old days you really had to get liability insurance from a company registered to do business in Mexico -- although your US policy will cover you from financial loss, the local law enforcement were reluctant to let you go as you were now a flight risk. If you were covered by a Mexican policy the other driver had somewhere to go to get paid.