Honestly, what strikes me about this line isn't even that Vader isn't Luke's father, but how wordy he is. I imagine they didn't spend too much time on phrasing and instead just tried to capture the gist with the expectation of polishing later, but even with the James Earl Jones's trademark slow, menacing Vader drawl, this just feels like it would sound weirdly verbose coming from Vader, especially in the middle of a fight.
Much of what you actually see in Empire is written by entirely different people.
> George Lucas initially hired Leigh Brackett, the sci-fi novelist who also wrote screenplays for Howard Hawks—including The Big Sleep (1946)—to write the sequel to Star Wars (1977). Brackett died in March 1978 while the film was still in pre-production, though, and Lucas wasn't satisfied with her script. Lucas wrote the next draft himself, which established structure and twists close to the final film, but suffered from dialogue. When Kasdan delivered his script for Raiders, Lucas asked him to rewrite The Empire Strikes Back. Kasdan suggested he read Raiders first, but Lucas reportedly said: "If I hate Raiders, I'll call you up tomorrow and cancel this offer, but basically I get a feeling about people."[9]
Brackett was a sci-fi author, Kasdan was a screenwriter by trade.
Despite seeming like similar tasks, screenwriting and book-writing actually have pretty distinct skillsets in some ways.
That wasn't Brackett's fault, Chandler famously said he didn't know who killed the chauffeur. Like many of his novels The Big Sleep was made by combining two or more of his short stories, and sometimes things didn't make sense.
Yeah, it immediately stood out to me how the final film leans so much more toward “show, don’t tell”. Vader doesn’t have to tell you how powerful he is! He just quips “Impressive” and goes on give Luke a smackdown. “All too easy.”
As a writer it's not uncommon to have a separate person (sometimes an editor) trim down/punch up dialogue. Dialogue writing is basically a separate skill from prose or screenplay writing, though the best writers are good at it too.
