Just reading what's been quoted in the blog post, this makes it so that an upsell purchase is a distinctly different transaction from a previously completed purchase.

What was possible before: 1. Visitor enters billing information and makes a purchase 2. Visitor is shown an upsell page 3. Visitor clicks a link on the upsell page 4. The item that was clicked is automatically added to the previously completed purchase

Now, step 4 isn't possible. A retailer must make it clear (by virtue of asking for billing info again) that there is a new purchase that's about to happen.