My understanding is that only human creativity can be copyrighted. So if you sketched out the plot and got the LLM to write all the words, then only the plot is copyrightable. So someone else can copy all the words, as long as they don't copy your plot.

However, as you point out, someone has to determine which bits the LLM created and which bits you created. If you wrote the whole book, and a tool incorrectly flags your writing as LLM writing, and then someone copies chunks of your book because they believed the tool and assumed they could (and assuming you filed a DMCA claim and they denied it using the tool's output as proof) then there's going to have to be a court case.

I suspect there's going to be a few court cases about this.