Even the "trash" hypothesis is just that - one of the theories of why this stuff was left behind!

Well yes, that's the beauty of it - the protagonist just makes a living hauling this stuff out of the zone(this is where Stalkers came from!), but in the novel there are various factions that believe all kinds of things - that these items are a gift from god, that they are trash, or that they are a test of some kind for us as a human race. I really do highly rate the book, it's great.