Yes for a walk-in customer to apply for an IDC you will need a bank guarantee, as stipulated in the application material.

Actually let me clarify: It's a banker's reference. Not a guarantee as such.

They want you to tell them who your banker is. Then they want you to sign an authorization for Amex to contact your banker and get information on your account. Your bank doesn't guarantee as such. There is no liability on your bank's part. Amex just want to know that your account is in good standing and that there's money coming in that loosely matches your stated income.