Is it coincidence? That's hard to tell.

Everyone involved in the production of the first three films has been extremely contradictory regarding how they were made. On the one hand, you have Kurtz, who said there was only going to be one film. On the other hand, you have Hamill, who said Lucas asked him to play Luke in a sequel trilogy and on yet another you have Lucas saying that he only intended there to be six films.

On the one hand you have Prowse spoiling Vader was Luke's father two years early. On the other hand, you have several others saying that Vader and Anakin weren't the same person until several drafts into the script of ESB. It really isn't much different.

If I had to hazard a guess, I'd say that the element of Vader as Luke's father probably already had made it into the script (since filming began in early 1979 and this is supposed to be from late 1978) and that Prowse simply caught early wind of it by luck.