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.
I don't even consider it a straight up lie. I know we have decades of this built into our culture, and its been basically ret-conned, but...
"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.
Pretty sure it was as much a part of planning the first episode as his later decision to have Anakin build C3PO. The early Star Wars scripts and materials are all over the map, there was no plan. This script just makes that all the more clear.
But the "I am your father" wasn't in the shooting script anyway. The script said "Obi-Wan killed your father", and it was changed to "I am your father when James Earl Jones" said the line for the voice of Vader. Maybe it is that way in subsequent published scripts but not the original.
"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.