Amex has run its own airline lounges for years – I remember using the Mexico City one about 7 years ago.
From the Amex website:
“Moreover, simply by showing The Platinum Card®, you will have access to the following American Express VIP lounges
• Mexico – Mexico City – Centurion Club
• Argentina – Buenos Aires – Salón Centurion
• Brazil – São Paulo – Centurión Club
•India- Delhi- Centurion Club