>Amazon's seller-side stickerless inventory documentation specifically says that identical items from different sellers are never put into same physical bin so that the original source can be traced.

Irrelevant to the discussion. Keep reading the link that cj provided, starting at the "Example", and you'll see that there is nothing preventing a counterfeit item being sold on behalf of a FBA seller even if the seller shipped genuine items to Amazon. All virtual tracking does is (theoretically) allow *tracing* of the counterfeit item should it be detected and reported.