Well, I understand, and I suspect that a lot of people commenting here see the term similarly to you; but there's an official definition regardless of your personal interpretation, and it does include the 'somebody else's work' part.
Why is translation a different beast? It produces text based on a prompt you give it, and it draws from vast amounts of the works of other people to do so. So if a translation tool does not change the 'authorship' of the underlying text (i.e., if it would have been plagiarism to copy the text verbatim before translating it, it would be plagiarism after; and the same for the inverse), then it should also be possible for an LLM to not change the authorship between prompt and output. Which means, copying the output of an LLM verbatim is not necessarily in itself plagiarism.
> but there's an official definition regardless of your personal interpretation, and it does include the 'somebody else's work' part.
No, it doesn't. First of all, dictionaries aren't prescriptive and so all quoting a definition does is clarify what you mean by a word. That can be helpful toward understanding, of course.
That said, the intransitive verb form of the word does not require "somebody else's work" in the sense of that "someone else" being a human.
> to commit literary theft : present as new and original an idea or product derived from an existing source
According to this, what it means is taking credit for a work you did not produce. That work did not have to be produced by a human, it merely had to exist.
> Why is translation a different beast?
Because it doesn't produce a new work, it just changes the language that work is expressed in. "Moby Dick" is "Moby Dick" regardless of what language it has been translated to. This is why the translator (human or otherwise) does not become the author of the work. If you were to run someone else's novel through a translator and claimed you wrote that work, you would in every respect be committing plagiarism both by the plain meaning of the word and legally.
> copying the output of an LLM verbatim is not necessarily in itself plagiarism.
Yes, it is. You would be taking credit for something you did not author. You would be doing the same if you took credit for a translation of someone else's work.