That's exactly what happened. There was no swapping with them on the cards (directly or via the buyer) as that creates risks since it's based on honesty, not protection through eBay. The only reason for knowing who the other seller was as the package I received had their original invoice for the item inside.

Both me and the other seller opened cases for eBay to step in via the return after the buyer admitted the mistake (they said the post office put the wrong return label on each box). Mine is still pending review but the other seller had the outcome today and seems to have been penalised for it.