"They keep all products with the same ID in one area"
No they don't. Google 'random stow' and you'll find articles about how Amazon stores incoming items. They are stowed somewhat randomly, but scanned as they are stowed. So the system knows where to find each unit again, and can use that to optimize picking paths.
By default, the system optimizes for speed and doesn't ensure that seller A's customers get the units sent by seller A and B's customers get those sent by B. This means that in a discussion about provenance of specific units, this is a distinction without a difference (or alternatively, all units of a product are in one area, which isn't necessarily physically contiguous in space).
Yes, my point is irrelevant to the main discussion about commingling.
I was commenting only because most people make the same false assumption (that all units of X in warehouse Y are stored together) and find the random stow model interesting when told about it.
No they don't. Google 'random stow' and you'll find articles about how Amazon stores incoming items. They are stowed somewhat randomly, but scanned as they are stowed. So the system knows where to find each unit again, and can use that to optimize picking paths.