I don't think this is true. I just tried to sign up and it says
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Payment/Identity Verification - You will not be charged
You won’t be charged for using OCI Free Tier. If you choose to upgrade your account to Paid tier, you will only be charged for services used over your Always Free limits. If you don’t upgrade your account to Paid tier, you will continue to have access to Always Free services.
You may see a small, temporary charge on your payment method. This is a verification hold that will be removed automatically. See the FAQ for more information.
We recommend double-checking all the information before submitting the form since some info is automatically filled.
Oracle does not accept any virtual, prepaid or single use cards in payment verification.
Debit cards that have a logo (for example, Visa, Mastercard, Discover, American Express) and don't require a PIN number can be used to sign up for a free Oracle Cloud Account.
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So it does require a credit card for sign up.