Exactly.
And, shipping prime items back to Amazon when they are sold and fulfilled by a 3rd party is not a benefit of prime. So, the fact that an item is marked as prime doesn't matter.
This is where your off a bit. When you say return back to amazon are you talking about the warehouse or the prime seller?
Let me be clear again. No matter the seller, if it states prime, even from a 3rd party, you receive all prime benefits which include shipping back to the original seller and a full refund. This has nothing to do with 2003 or orders made back then or even last year. This something new that amazon is doing to expand their prime sales. Started a few months ago.