The story is post-apocalyptic because something terrible has happened that they call the Great Burning.
Since they call this event the Great Burning, we can imagine it was some kind of nuclear holocaust. Also, they refer to looking for metal in the Place of Gods. Eventually, the reader learns that the Gods were us, and the place John is going to is a city.
It is forbidden to go to any of the Dead Places. … It is forbidden to cross the great river and look upon the place that was the Place of the Gods … It is there that spirits live, and demons—it is there that there are the ashes of the Great Burning.
This is how we learn that John is a in a post-apocalyptic world, where there were once humans living in houses and cities like we live in today. He talks about priests like his father going into the houses to search for metal and seeing bones that are turning to dust.
After a time, I myself was allowed to go into the dead houses and search for metal. So I learned the ways of those houses—and if I saw bones, I was no longer afraid.
John’s people think that the people are Gods, but they are not. They are the humans who lived before the apocalypse. There was some kind of nuclear event called the Great Burning. When John goes into a house in the city and sees one of the “Gods” in a chair “watching his city die” he knows the truth.
That is all of my story, for then I knew he was a man—I knew then that they had been men, neither gods nor demons. It is a great knowledge, hard to tell and believe. They were men—they went a dark road, but they were men.
John goes back and tells everyone, and they decide to take a company back to New York city (“newyork”), to check it out. He is glad that they have learned the truth.
This story, like many post-apocalyptic tales, tells us about a group of humans who have tried to make sense of the world as they see it. They have interpreted the world based on the clues they have, archeologically. They idolize us because to them we seem like gods, and they do not have enough information. We seem so advanced, based on what they are capable of. Yet we destroyed ourselves. Now that they know the truth, hopefully they can avoid our fate.