Imaginative reconstruction, and a lot of reading and watching pre-1977 books, comics, and movies.
In which case, it’s very impressive indeed!
I was simply under the impression that the original “Journal of the Whills” was a first-person two-page setup story. I’m working on a transcript of the single released page, and it is pretty interesting. Though if I’ve learned anything by looking at both that and the Empire Strikes Back treatment; Lucas is a terrible speller.