Mostly wrong. Amazon does not put all the inventory together into one bin, but they explicitly tell FBA sellers that they will ship a different seller's product with the same SKU if it is in a warehouse closer to the buyer, for example. Then, they make up for this by taking one of your product and putting it in the other seller's inventory. Thus inventory is commingled, both at shipment time and then in the inventory to make up for any commingling that happened at shipment time.