Your missing the key part. You can order from the manufacturer through amazon, and they will ship you the part from the 3rd party (that may or may not be legitimate) if the have the same ID number. Even though you ordered from the manufacturer, you may get a fake product. They commingle inventory, to make it easier for them to ship from nearest warehouse, but don't really put enough work into ensuring the products are actually identical.
The funny thing is Chinese e-shops don't do that. You can find a bunch of the same goods at e.g. JD but if you buy from the JD itself ("Joy Collection" in the English JD frontend joybuy.com), it would never be fake and it would never be co-mingled - and the reviews are separated too.
They combine inventory for the same item from different sellers. Even if the item you bought says it is being sold by Amazon that doesn't mean that the product you receive will be one that they sourced. It could easily be from a third party seller instead. Unless you're buying something like they're own Amazon Basics brand you have no way to reliably tell where something actually came from on Amazon.
>Doesn't amazon make it fairly obvious when you're ordering from a 3rd party or not?
I would say "no," because pretty much everyone I know thinks Amazon is a store with a single source of goods, not a "marketplace." This is probably intentional.
It’s actually worse than you think. Amazon often stores their “sold directly by Amazon” inventory with “fulfilled by Amazon” product sold by third party sellers. This would make getting a knockoff just a matter of bad luck, even if you order from Amazon themselves.
I haven’t ordered from Amazon since I learned this months ago, with two exceptions for birthday presents.
Yes it makes it clear (but they can "commingle" the same product from different sellers, as said in the related comments).
However let's say you want to buy a product. But the reviews are mixed. Some are 5-star reviews, but there are 1-star reviews as well. And there are 5 sellers. From which one do you buy? All the reviews are mixed.
Perhaps a bad review is caused by a seller sending fake product. But perhaps the review is a valid review and it highlights a defect in the original product. You can't know.