Apologies if I’m misunderstanding, but are you implying that amazon doesn’t commingle products? Allegedly this is the reason that you can order a “sold by amazon” product but still receive a counterfeit item (likely provides by a 3rd party seller).
In essence commingling prevents you from efficiently grabbing something from a specific seller, but it doesn't prevent you from knowing which one you grabbed.
And do they share that knowledge with you? Or only after you've gone to the trouble of returning the item?
If the unit has an Amazon seller-specific FNSKU barcode, the unit is not commingled.
The used barcode type can be configured by the seller.
The barcode configuration option does not mention commingling at all (but help pages do), so I can understand some sellers getting confused on whether they are affected by commingling or not.