To be brief, it is a short short story and not a prose poem because it has at its center a character who yearns.
Fiction is a temporal art form. Poetry can choose to ignore the passage of time, for there is a clear sense of a poem being an object, composed densely of words, existing in space. This is true even when the length of the line is not an objectifying part of the form, as in a prose poem. And a poem need not overtly concern itself with a human subject. But when you have a human being centrally present in a literary work and you let the line length run on and you turn the page, you are, as they say in a long storytelling tradition, “upon a time.” And as any Buddhist will tell you, a human being (or a “character”) cannot exist for even a few seconds of time on planet Earth without desiring something. Yearning for something, a word I prefer because it suggests the deepest level of desire, where literature strives to go. Fiction is the art form of human yearning, no matter how long or short that work of fiction is.
James Joyce spoke of a crucial characteristic of the literary art form, something he called the epiphany, a term he appropriated from the Catholic Church meaning, literally, a “shining forth.” The Church uses it to describe the shining forth of the divinity of the baby Jesus. The word made flesh. In literary art, the flesh is made word. And Joyce suggests that a work of fiction moves to a moment at the end where something about the human condition shines forth in its essence.
I agree. But I also believe that all good fiction has two epiphanies. There is the one Joyce describes, and there is an earlier epiphany, very near the beginning of a story (or a novel), when the yearning of the character shines forth. This does not happen in explanatory terms but rather is a result of the presence of that yearning in all the tiny, sense-driven, organically resonant moments in the fiction, the accumulation of which reaches a critical mass which then produces that shining forth.
And because of the extreme brevity of the short short story, these two epiphanies often—even typically—occur at the same moment. The final epiphany of a literary short short is also the shining forth of the character’s yearning.
It has been traditional to think that a story has to have a “plot,” while a poem does not. Plot, in fact, is yearning challenged and thwarted. A short short story, in its brevity, may not have a fully developed plot, but it must have the essence of a plot: yearning.