Fortunately, even with Ben sounding like he's referring to Vader and Anakin as 2 separate people, it still works per the overall story because Obi-Wan and Yoda both felt Luke needed to find out that truth at a certain point and ANH was far too soon as even in ROTJ, Yoda says of Luke in ESB, "then, not ready for the burden were you." If Ben were to speak of Anakin's death more faithful to the truth, Luke may have picked up on it, but by being coy, Luke knew enough to know Anakin was a good man, Darth Vader in some form was who killed Anakin, and that's all.
Here's where we get into the aspect of discussing STAR WARS on an 'interior level' or an 'exterior level'.
Interior level = "When Ben said this, he actually meant..."
Exterior level = "When George Lucas wrote Ben saying this, he actually meant.."
The two discussions are mutually exclusive. It's hard to discuss one when the rebuttal addresses the other.
You wrote "If Ben were to speak of Anakin's death
more faithful to the truth, Luke may have picked up on it, but by being coy, Luke knew enough to know Anakin was a good man, Darth Vader in some form was who killed Anakin, and that's all".
That's a great rationalization to explain what was going on, from the perspective of an interior discussion of the story.
My point is, from an exterior perspective (which is where this discussion has led) you address what the creators were motivated to say and do.
The "truth" that Ben is referring to actually changed from 1977 to 1980. When Alec Guinness was given his script to memorize in 1976, Luke's father was killed by a bad Jedi named Vader; that's the extent of the Skywalker backstory. Since Ben's story to Luke is vague enough and free from details, when the filmmakers decide to change the 'truth' from "Luke's dad was killed by Vader" to "Luke's dad IS Vader", all the audience has to do is accept that Ben was keeping the truth from Luke for his own good.
Guys, I'm not trying to be a tool or anything. I'm just saying that it's A-OK to discuss retro-continuity character motivations, and what an OT character was actually 'thinking' now that we have PT events to compare them to. But the poster was actually adressing the fact (and it is a fact, a documented one) that Lucas changed his mind (which he has every right to do) after STAR WARS and before EMPIRE as to what the 'truth' of Ben's story was. Especially since that story change really doesn't require Ben's ANH tale to Luke to be altered; it just requires the audience to have a change-of-context in their perceptions.
The bottom line is: Lucas didn't have all of this written in stone from Day One. It's fun to come up with rationalizations to make it all work, but STAR WARS was a work-in-progress the whole way. Some stuff was concretely figured out early, some stuff was thought up down-the-road, and required some deft screen-writing and/or audience suspension-of-disbelief to blend seamlessly.