Wow, poor advice! Who do you think will win that battle when AMEX commences debt collection - AFTER downgrading the credit rating?
Not poor advise!
AMEX may start debt collection. It means nothing. AMEX does not have special powers. It is a private entity.
At least in the EU the cardholder can dispute the claim - and then it is practically forbidden for the bank to downgrade the credit rating - unless the bank receives a court verdict and the cardholder is not paying after that verdict for 4 weeks.
There is no way the OP can refuse to pay what is owed to AMEX on the grounds that the OP believes himself entitled to a chargeback.
There is an airline, which did not run the flight - and which is obliged to refund the flight costs within 7 days - the airline is presenting bogus evidence to fight the chargeback - - and Amex has already initiated a chargeback.
Again, I do not believe that Amex will be very interested in contesting this in a court of law.