This is true. But at the same time Amazon tries very hard to hide any information about the customer from the seller. If the order is fulfilled by FBA, the 3rd party seller has the name of the customer and that's it. Amazon also can facilitate email exchange through their server, but most customers do not see it (getting into spam?) or ignore it. So in practice, you rarely have a reliable channel to communicate with your "customer".
Compare to eBay - you have all the info for the customer, can follow up, resolve issue, etc. On eBay there is no confusion about who is "the seller".
I understand why Amazon is doing that - theirs customers probably hate idea of dealing with individual sellers and instead prefer the uniformity of dealing with Amazon. But then the position that Amazon is not "the seller" kinda weak.