Sounds like you got an inexperienced support person. My first advice would be to call back and hope you get some familiar with the approval process and the issue in general.
Apple is not a bank. Apple does not actively participate in the approval process. When you enter card details on your iPhone, the information, along with personal information on your device, are encrypted and sent to the Payment Network Operator (Visa, MasterCard and American Express). The PNO decrypts the data, reviews it and if they validate the information, pass it to the bank for final approval. If the bank approves the account it creates an encrypted token. The token is then sent to Apple servers and installed in your Apple Wallet app. If the token is not received in the allotted time, you receive an error message that the card can’t be added and contact the issuer. Again, Apple does not approve or decline adding cards to Apple Wallet.
American Express is both the account issuer and Payment Network Operator. Their support team is usually aware of what’s going on and works with their clients to resolve any outstanding questions or concerns and then issues a token.
You can review how similar issues have been successfully resolved by other community members in the past.
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I’m happy to answer any questions or concerns you may have.
Apple Pay security and privacy overview - Apple Support