As we learned from Hamlet, any time you see a small story contained within a bigger story, it's a good idea to pay attention. That's because this smaller story, generally referred to as a "play-within-a-play" (even when it's not a play), usually tells us a lot about what's going on in the work as a whole. In fact, it's usually an allegory: an extended metaphor in which objects, actions, and characters within the narrative are equated with meanings that lie outside the narrative. Think Animal Farm and you'll get the picture.
In "The Secret Miracle," what we have is a "play-within-a-short-story." Though we don't ever see The Enemies performed, the narrator offers us a summary. And that's one important summary: it reveals that the play's protagonist has a lot in common with Jaromir Hladik (Borges' protagonist). Not only do they share similar names, but they also experience time in a similarly strange way.
In fact, when we discover that the entire play exists only in the protagonist's mind, we get a little foreshadowing clue about what Jaromir will experience at the end of our story. The very act of writing the play exists only in Jaromir's mind: no one around him has any idea about his year's work, just like no one in his play is aware of the protagonist's fantasy. This is definitely not a coincidence, we're sure of that one.