Huuuuge citation needed.
It’s precisely because of comingling that they don’t have the ability to do this: they put multiple sellers goods, including their own, in the same storage bins, provided they have the same UPC. By what means do you suggest they can distinguish Seller A’s widget in Bin 37624 from Seller B’s? They do not appear to attach any additional tracking stickers to their goods.
And they do not have a formal system for complaining about counterfeit goods. They don’t even offer it as a distinct option on returns — probably quite purposefully. All you can do is request a return for some vaguely-related reason and manually add a note about it being counterfeit. They’re clearly not attempting to automate identification of bad goods, let alone removal of bad actors.