How did John figure out the "gods" were human just by looking at the dead man who was staring out at the city?

John looks at the dead man, preserved like a mummy, and sees he has dry skin like leather, is neither young nor old, and wears a look of sadness and wisdom. John doesn't explain how he knows this figure, though dressed in clothes of a god, is a man, but we can surmise it is because the corpse looks so obviously like one of his own people that John is unable to mistake it for anything but a human. Further, one imagines a god would not die.
John says this of the dead man he finds:

He was sitting in his chair, by the window, in a room I had not entered before and, for the first moment, I thought that he was alive. Then I saw the skin on the back of his hand—it was like dry leather. The room was shut, hot and dry—no doubt that had kept him as he was. ... He was sitting looking out over the city—he was dressed in the clothes of the gods. His age was neither young nor old ... But there was wisdom in his face and great sadness. You could see that he would have not run away. He had sat at his window, watching his city die—then he himself had died. ...
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 possible that John is the first of his people to see a fully preserved human body from the old times. The rest of his people might have conjectured that the beings who built such a marvelous civilization were godlike, but now John knows differently. Because he recognizes that the people who built this advanced society were humans, he realizes his own people can also accomplish the same feat.