But for a lot of the items, the only sellers are official.

Look at the Apple Lightning cable, a typical fake product, here in the UK the only place you can get it from is amazon or amazon warehouse, so what products are there to get mixed up? Again, this seems to be more of an Amazon US issue, the reddit post you linked yet again just uses US sources.

I've not experienced this with Amazon, even when buying highly faked items, items that i have purchased from ebay however have been clearly fake. Sure I could have missed a fake product or two, but i've not been obviously scammed where it matters, but on ebay this has happened pretty much every time I buy a highly counterfeited product.

Apple specifically has deals with Amazon such that Amazon is the only allowed seller and there is no third party inventory to commingle. Most brands do not do this, apple is an exception and has the clout to demand such an exception.