Fantasy as reality?

The Song of Achilles by Madeline Miller is a really interesting example of the problem of perception in fantasy.  It's a retelling of the myth of Achilles, from his boyhood to his fate in The Illiad, and is told as a faithful, realistic story, as it might have been told by an ancient Greek with access to modern storytelling methods.  So it includes all the elements of Greek religion, which are now considered Greek myth; most importantly to this story, the gods are very present, and the belief that they can effect everything that happens to humans, if they care to, is absolute. All this is handled in a very matter-of-fact, concrete manner.

This is an example of one person's religion being another person's fantasy.  What was very real and evident to the Greeks, supernatural beings needing to be worshiped, feared, and appeased, is to us nothing but a primitive society's way of explaining things that we feel either have or need no explanation, or that can be explained by science.  So what's fantasy to us was deadly reality to them, and this is one of the aspects that makes fantasy such an interesting genre, and so potentially tricky.  Because sometimes the alternate life of these elements we see as fantasy is not in the distant past, but here in the present, living cheek by jowl with the majority hard-headed view of reality in the modern West. And who's to say what's ultimately the truth?