What I meant (or I've meant, I'm a bit drunk as of now) by "disregarding the prequels" is that the storyline, "as is" in 1980 gives you a lot of choices as to why Luke doesn't know the whole thing. If we are to go with the info given in ANH, all Luke knows is:
The Clone Wars was serious business, Jedi knights fought.
His father participated, so did Obi-Wan.
Darth Vader is a former Jedi that killed his father.
Notice how "Anakin Skywalker" hasn't been written yet.