I have added ajnaro's question to the Primary CDW discussion thread.

The distinction between primary and secondary coverage is only important if you have other insurance which applies to the rental. Most USA-issued auto insurance policies provide no coverage outside North America, with the result that your credit card coverage becomes your only insurance.

However, bear in mind that this only covers damage to the car itself. If you kill or injure anyone, or damage someone else's car or other property, there is no protection provided by most credit card policies.