That short is essentially the creation of the universe, with each entity representing Day and Night (though they switch between it - like The One Above All and The One Below All being the same person, but two separate personifications). Since they clearly just met, I'm assuming it's the first day/night, with the ending being the first sunrise/sunset and the creation of the Pixar universe.
As for the human references, fate is already established in Brave, and Monsters Inc. has time travel, so I'd say we can assume it's a window into the future, or a personification of humanity, one of the 3 leading forces in the Pixar Theory's overarching story.