Commingled doesn't mean they're in the same bin, it just means that they can be used to fulfil other sellers' sales.

I have inventory with no sticker on it in NY, you have inventory with no sticker in CA. Someone orders from me, they live in CA, amazon takes yours off the shelf and sends it to them. Amazon still knows that that sale was fulfilled from your inventory.

Same thing with multiple bins in the same warehouse. I'm told by people who've toured warehouses that bins are kept separate and everything is tracked.