It's worth reading the essay. Plagiarism isn't always blatant. It can be subtle or accidental. Coincidences can look like plagiarism. Publishing plagiarized material is embarrassing and reflects badly on the magazine and its editors. Openly accusing an author of plagiarizing material can damage the career of an author who doesn't deserve it, with the risk of being sued and having to prove it. Authors and editors generally want to avoid even the appearance of plagiarism.
I also found this 1976 interview with Isaac Asimov where he talks about the line between acceptable borrowing and plagiarism:
...the general feeling is that ideas are common property: if one SF writer thinks up something which is very useful, another may put it into his own words and use it freely. Nobody in SF is going to accuse any other person in SF of using his ideas; in fact, we borrow so generously that there's no way of telling whose idea it was originally. For instance, in my novel The Caves of Steel, it was very important to the plot to have moving sidewalks, with an elaborate system of side strips that enabled you to work up to the speed of the sidewalk or to work down to the surrounding, motionless medium. This had already appeared some years before in Heinlein's "The Roads Must Roll." Well, I borrowed it without any worry at all. I'm sure that Heinlein in reading my novel would have recognized his system, but who knows where he got it from? He never said anything. It'd be different if I used the details of his plot, if I worked up a story that was so like his that nobody could fail to see it—that's plagiarism. But just to use the idea and build your own plot or story about it—why, we do that all the time. And they do it from me, too—you know, they use the three laws of robotics—and they're welcome. I have no objection.
The magazine apparently received a number of letters from readers which they decided not to print, likely to avoid publicizing the incident and publicly accusing the author. They wanted to respond to these letters, to explain how it happened, but they couldn't get too specific without again accusing the author. So they published this essay, which should be viewed in part as a response to the incident and those letters. The last two columns of the essay can be read as an explanation of what happened, and what the magazine has done about it.
In this light, one story stands out as a likely candidate: Dèjá Lu by Henry Clark. The story is a good circumstantial fit:
- It was published in November 1984. The essay was published in August 1985, and the letters column from the August issue mostly discusses the January 1985 issue, giving some clue to the publication lead time.
- This was last work of the author published by the magazine (previous works are 1, 2, 3). In fact he has no further credits in ISFDB until some childrens' books thirty years later. He apparently also wrote for Mad magazine and there's a gap in his contributions covering this time period, for whatever that indicates.
- The story has never been reprinted.
- I could only find one letter which mentions the story, in the September 1985 issue, by a reader apparently trying to say something about every story in the issue:
"Deja Lu” by Henry Clark should become a classic — the P. D. Q. Bach of science fiction; plagiarism with humor is the sincerest form of flattery. Or something.
(Spoilers for Dèjá Lu follow.)
Reading the story, it's clear that it might have a plagiarism problem. Dèjá Lu is chock full of references to other stories, plagiarism is mentioned in the second sentence, and the title "Dèjá Lu" literally means a feeling of having read something before.
The story's conceit is that the narrator had something implanted in his brain which causes him to confuse fact and fiction. As he tells the story, he repeatedly includes bits from other stories. Many of these references are obvious, notorious, and don't affect the plot. The editors apparently felt okay with passing references like James Bond, Tralfamadorians, and "To Serve Man". Some other borrowing is less obvious, however, and may have slipped past the editors. Then it was viewed as crossing the line once readers called their attention to it. Here are a few that I've identified:
- The passage "It was on a bitterly cold and frosty morning during the winter of ’97 that I was wakened by a tugging at my shoulder...." (page 128 in the magazine) comes directly from a Sherlock Holmes story.
- The line "As a young man I had resolved to forge in the smithy of my soul the uncreated conscience of my race." on page 129 is from a James Joyce novel.
- The alien name "Egtverchi" is from the James Blish novel A Case of Conscience.
- The term "Polywater" is from some 1960s junk science. But the term "Ice-9" and the idea of a substance destroying the world's water supply comes directly from Vonnegut's Cat's Cradle.
- The idea of scaring away the aliens by telling them tall tales comes directly from a Twilight Zone episode (acknowledged in the story).
- A story referencing its own publication has been done at least once before: Don't Look Behind You by Frederic Brown (skip to page 56 column 2 if you don't want to read the whole thing).