the promo code functionality should not be added to the confirm page.
it should be added to the cart display page.
There are a number of behaviors on the confirm page that trigger on any form post and are intended to happen when the checkout form posts to the confirm page. if you are to add the promo code functionality on the confirm page, you would need to update all of those behaviors to use the Checkout form submit trigger instead. I recommend not adding it to the confirm page but to the cart display page instead. The PDF you refer to adds the functionality to the cart display page, not the confirm page.