yeah I think there is a narrow legal meaning of plagiarism where use of public domain works doesn't fit, but theres a broader understanding of plagiarism. Outside of a school context, If somebody uploaded a short story that was a very light edit of a public domain work, under their own name. Have they committed plagiarism? Legally speaking no. Have they still done something shitty in taking credit for someone else's work? Yes. Does it make sense to call this plagiarism? Maybe? Do LLMs fall afoul of this broader understanding of plagiarism? Arguably yes.
It's a sort of "taking credit for others' work" sense of plagiarism that I think still applies.