This script is good evidence they made up the connection part way through Empire. Also, if Lucas already had it in mind for Star Wars, I doubt he'd have Obiwan straight up lie about it.
"A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi Knights. He betrayed and murdered your father"
I could still see Ben making this metaphor for the internal struggle Anakin dealt with, not wanting to reveal the truth in that setting.