Exactly. This isn't a usability improvement, it's fluff.
If you want to see an actual usability improvement, look at what Stripe has done with jQuery.payment: https://github.com/stripe/jquery.payment A good example is in their Checkout javascript component: https://stripe.com/docs/checkout
I should also point out that this seems to be just one part of the checkout process, so there are cases where it would not work (though that's probably not the intended use-case for modal form).
Do you have any data that backs that up? (excluding call-to-authority and anec-data)
Although I understand that its the norm to ignore previous computing history for the illusion of inventing something, more so if it's pretty.
No one has tested this form. So just putting it down seems premature.
I'd prefer people to be data driven, than just cargo culters.