I've been ordering on Amazon in Germany for a good 20 years now and I've never received a counterfeit item. Is it not a thing here? Does it primarily affect certain countries? Am I insanely lucky?
I think I'm pretty good at spotting fakes, because I'm sensitive to tiny typographical or material-wise quirks. In the same period, I've received multiple fakes on eBay, including a genuine phone that came with a counterfeit charger.
I can imagine that commingling introduces a very low-percentage risk of receiving a counterfeit product but due to the immense scale, it still affects a huge absolute number of orders.
I think what a lot of people qualify of counterfeit (not saying OP does here) is people buying cheap no-brand Chinese garbage and receiving cheap no-brand Chinese garbage and not being happy with it.
But that's the other aspect to commingling, I guess - you might pay full price for the real deal and get a fake, due to bad actors' stock being commingled, but on the flipside a punter who's paid an unrealistically low obviously-fake price might actually get the real deal, adding an air of legitimacy to the bad actor.