It also (rather uncomfortably in light of later revelations) features Luke & Leia as a romantic couple! Apparently Lucas signed off on Foster's manuscript, so he hadn't had the idea that they were siblings yet.
Luke and Leia were teased as a romantic couple in Empire, she kissed him on the lips to draw Han's ire, and then it's hinted that she's his sister in the final scene of Dagobah/when he calls out to her via the force. The incest thing is something that Lucas was deliberately playing with, whether Splinter took that into account or not.
The comment you’re replying to is correct; there’s plenty of evidence that Lucas made up both the plots and crucial details of all the Star Wars films as he went along, rather than plotting it all out in advance. Telling people he had, or at least allowing them to believe so, was just another inspired marketing trick.
