I believe your description was accurate before ca. 2015, but at some point they reportedly started making commingling the default even for "Sold by Amazon" items. Commingling isn't the only issue, either; there's still the problem of products for which Amazon was never an authorized retailer and so their listing/SKU may have been created to sell counterfeits in the first place. In any case, there have been multiple lawsuits specifically alleging that counterfeit items were "Sold by Amazon" [1] [2] [3].
[1] https://web.archive.org/web/20201125141347/https://www.forbe...
[2] https://www.courtlistener.com/docket/4495986/1/apple-inc-v-m...
[3] https://www.geekwire.com/2016/daimler-ag-sues-amazon-knockof...