There are other vectors for counterfeiting own brand products but you would have to be a supplier. In normal retail 'own brand' products can be sourced from many suppliers. If you are selling ready meals then the exterior pack could be provided by the supermarket with that being consistent. Meanwhile the contents could come from one of many industrial units at one of many suppliers. In this way 'co-mingling' is possible. In the UK we had a little problem a few years ago where 'own brand' ready meals were full of horsemeat. The products themselves were 'legit' but the contents were not.
I would say that Amazon Basics are vulnerable to this type of fakery that goes on further up the food chain. This is fully possible with commodity items such as batteries. As per the UK horsemeat problem there is a serious reputation risk if this happens, terrifying the customers this way is a bigger existential threat to the brand than any 'anti trust' laws.
It would be interesting to see counterfeit numbers for items sold by Amazon itself vs by someone else on Amazon. Anecdotally I've never had an issue with stuff sold by Amazon itself and steer away from our vendors if possible.
At some point this crosses the line into anti-trust territory.