Thanks for submitting this.
An important distinction is that Ontraport's order forms are specifically designed for transactions where payment information is being collected. Applying a 100% coupon changes the amount being charged, but it doesn't change the form from an order form into a non-payment form, which is why the payment fields remain required.
I took a closer look at your use case and there is another way to create the experience you're looking for without requiring a credit card. I recorded a short video showing how to use a regular form with your special code, grant access to the workshop, and even record the $0 product in the contact's Purchase History.
I'll leave your request here since the specific behavior you're asking for: allowing an order form to dynamically stop requiring payment information when a coupon brings the total to $0 though.