I heard about that, but given that he then went on to say that Luke would turn to the dark side, I think he was joking. He was using that twist as an example of a ridiculous plot point that could never happen, not knowing that they were actually planning to use that twist.
This is all it said in the article I had. I feel like he was serious.
"Darth Vader and Luke Skywalker…are hooked up in a do-or-die light saber duel when Luke learns that Darth is, in fact, his long-lost father. “Father can’t kill son, son can’t kill father,” Prowse said. "So they live again in “Star Wars IV.” (Wild cheers.)