In the manga, Sailor Cosmos implied that she was a future Sailor Moon. She was telling the Asteroid Senshi about how she remembered the battle with Chaos (the current battle in which Eternal Sailor Moon was fighting). Cosmos states that she was coward who couldn’t do the right thing, and after the battle, she often wondered what would have happened if she made a different choice. So she traveled back into the past in the form of Chibi Chibi to help Usagi (her past self) overcome her loneliness and make the right choice. During the battle with Chaos, Chibi Chibi revealed who she really was.
Chibi Chibi doesn’t become Sailor Cosmos, Chibi Chibi is Sailor Cosmos. And if Sailor Cosmos is a future form of Sailor Moon, as the manga heavily implies, then that makes Chibi Chibi a future form of Sailor Moon as well :)