Upvoting because this is so beautifully executed and I'd love to see people test this out on their checkout process. That said, my experience indicates that this is not going to convert well. To mirror the author's own quote: "Every question a user has to ask themselves during the checkout process is another reason for them not to complete it." What's presented here is a drastically different experience from the norm…
This actually happened when I refreshed the page while playing with it and it was replaced by a normal form. Pretty handy. I think I will also be testing it out.