I think it is somewhere in between. All the things you listed are true, but Amazon will still say things like "Sold by ...", and there is still the option for drop shipping (or something very close to it).

I am fairly sure that if Amazon had started the marketplace by promoting it just as an online department store, with all the "selling to retail" elements in the GP, it would have been much less successful than it was given their actual promotion of it as a marketplace.