You're not understanding how comminging works here.
Say I'm shady seller X, I'm sending in fakes to FBA (lol). Yes, I'd like to sell "Product Y."
I'll list Product Y and enable "commingling," and send in fakes.
Now when Amazon sells Product Y, Amazon will choose an item from it's shelves in a warehouse. Since shady seller X said "commingle my inventory" Amazon may choose the product(s) shady seller X's sent from it's shelves.