Been ordering since January 2000 (> 1,000 orders) from Amazon Germany and never received anything counterfeit as far as I can tell.

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.