The story was a work of what's now called "Flash fiction," because it tries to convey a full story in as few words as possible. Hemingway's story is this: "For sale: baby shoes, never worn."
The 6 word story is an extreme example of Flash Fiction. There is an award called the Micro Award that recognizes the best stories that are less than 1,000 words long. It's called the "Micro Award."