the 13 page summary he's talking about was his first story treatment for "Star Wars." Meaning his "backstory" was just a pre-rough draft version of what became Episode IV. He didn't write it to be a backstory, after refining the screenplay, he realized he could use that rough draft as an idea bank, essentially.

He didn't even come up with the idea of making Darth Vader = Anakin Skywalker until during story ideas for ESB. And ESB was titled "Chapter II: The Empire Strikes Back" for the first year of pre-prod.