This is oft-repeated mythology that became popular on reddit; it's part of the campaign of feminist revisionism that's commonplace over there regardless of whether it matches reality or not. They constantly try to argue that it must have been a case of a man getting credit for work a woman did, even when the woman concerned made no such claim or even received appropriate credit.
I think it's justifiable to call out her contribution. She was an excellent editor without Star Wars, her work includes American Graffiti and Taxi Driver.
I consider her a part of what makes the original trilogy great but I don't think it diminishes Lucas' contribution. No one person is "what makes it great". Plus the internet loves the underdog or a a "TIL"
yeah like those nags Marie Curie and Margaret Keane
She won the Oscar for editing "A New Hope"
I don't think its mythology to claim that Marcia Lucas edited the films. It's pretty well accepted reality.
If the remasters and the episodes I-III are anything to judge by, George really does better when there is an editor that will check him.