I don't think you have to be US citizen. Anyone with a SSN can apply. Plus the Platinum card is offered in many other countries.
Here's the US versions T & C:
TERMS AND CONDITIONS
By submitting this application, you are requesting us to open an Account in your name and to issue Card(s) as you direct. Only qualified individuals 18 or over may apply for an Account. This offer is available to US Residents, excluding Puerto Rico and the US Virgin Islands.