My first theory was that the solitary sentance was taken from a treatment made in preparation for the rough draft, for various reasons. But it was actually a 2-page treatment Lucas made before the May 1973 hidden-fortress-inspired treatment. He seems to have written it at the very end of January, 1973. This comes courtesy of Rinzler's Making of, which is the first actually validation of the treatment beyond the one sentence that existed beforehand. Annotated Screenplays nonetheless seems to have been written in ignorance to the document, so I assume Rinzler was the one who discovered it. He was probably tipped off because Lucas talks about its content in the "Lost Interviews" that form the basis of that book.

EDIT

See here:

http://www.secrethistoryofstarwars.com/journalofthewhillspart2.html