I know, I’ve heard the story.
I don’t think he was thinking of Sequels at the time.
It’s hard to say as Steven Spielberg said in a 90’s BBC special that part of George’s concept for the Prequels was the Sequels. He also slipped up in an interview during the lead up to Attack of the Clones where he mentioned story points in II have to connect with VII.
I think some of it was left open to the possibility but not too much in case he never did. That’s why things were addressed in Labyrinth of Evil at first and then another time in Darth Plagueis novel. There would be some kind of explanation but not necessarily what he had in mind until he addressed things in Clone Wars with his explanation that would be added to and finally come full circle in his Sequels.