I am an Amazon seller, and yes if you ship inventory to them under FBA program, that inventory is co-mingled.
For new products, that is how Amazon does it. Which will explain why many products will have inconsistent reviews about fakes. And you can ignore any review that says "Avoid seller X. He sent me a fake!" You have no way of knowing which seller's item you got.
Note that this is only for "Fulfilled by Amazon" orders. For non-FBA, the seller ships it to you directly.
And for used items, Amazon will ship you the exact one from the seller.