Personally, I’m more inclined to believe Marcia Lucas here.
2012 is rather late to spring that kind of anecdote though, wouldn’t you say? In order to add ‘suspense’ to the story/plot , all Lucas would need is Vader to claim to be Luke’s father. It doesn’t have to be true on a deep-below-the-surface story level. Huyck could make the suggestion, of course: “hey, have Vader claim to be Luke’s father”. But Huyck saying “make Vader Luke’s father” is overreaching. EMPIRE (ESB) is transitional either way - it is not the origin point of the twist. The twist either originated with JEDI, or with the first film, STAR WARS (ANH). Any honest assessment of EMPIRE not colored by hindsight, would see that it’s not conclusive either way in isolation (whether the film or the various scripts/drafts).
It really wouldn’t make sense to invent the whole “from a certain point of view” retcon if Vader was always supposed to be Luke’s father.
Actually, it does, for several reasons. The “certain point of view” came about because: One, Kasdan thought that Lucas needed to give the audience an in-story reason why Obi-Wan told Luke what he did in the first film, not because he (Lucas) needed to give the audience a reason why he (Lucas) told them one thing and not another. Two, because come JEDI, the premise for the Father Vader twist had changed: this is where Luke says that there’s still good in Vader. I don’t think this was in the first two drafts that were written solely by Lucas. Three, “He betrayed and murdered your father” was a cover-story from the get go. One, it appears rather late in the story/scriptwriting process for STAR WARS - it’s in the very last draft. That and the fact that this element appears some almost three months AFTER Lucas told collaborators in a late December 1975 meeting: “in the next film/story (the sequel), the audience will find out who Darth Vader is”. This last bit, is likely pointing to the earlier version* and origin point of the twist, that Luke’s father had taken on the identity of Darth Vader (whom he had killed) for noble intentions perhaps, but which had consequences later on. This is very aligned with 1970’s Lucas and Kurtz and their mutual admiration for Kurosawa’s films and the moral message that accompanied them. With JEDI, Lucas changed this to: Anakin (Luke’s father) had turned to the dark side. And here too, likely Lucas added the redemption subplot, which is largely based on Luke’s somewhat baffling retroactive perception of Vader’s behavior and actions in the previous film that lead him to believe that Vader was acting out of some ‘buried’ goodness. Essentially, Luke - and Lucas - was retconning EMPIRE’s Vader. In the 1983 “From Star Wars to Jedi” documentary, Lucas only claims that Vader being Luke’s father, and the helmet coming off were part of ‘the original story’. He doesn’t say that the redemption part was part of that story originally.
*the subplot from the PT about Dooku and Anakin replacing him are likely echoes of this older idea. Same with the idea Lucas had for the second draft of TPM, where Obi-Wan the master is actually killed, and his student Qui-Gon takes on his name and identity thereafter.