The point of steganography (as discussed in the paper) is not unerasable watermarks but undetectable (to the adversary) messages in innocent-looking communication.
I'm confused why you focus on plagiarism detection. That being said, your scenario is very briefly mentioned in the conclusion and requires augmenting the approach (entropy coding) with error correction.
The result would be that as long as your modifications (reordering clauses, etc.) reasonably closely follow a known distribution with limited entropy (which I think it clearly does, although specifying this distribution and dealing with the induced noisy channel might be very hard), there will be a way to do covert communication despite it, though probably only a very small amount of information can be transmitted reliably. For plagiarism detection, you only need a number of bits that scales like -log[your desired false positive rate] so it would seem theoretically possible. Theoretically it also doesn't matter if you use text or images, though in practice increasing the amount of transmitted data should make the task a lot easier. However, I'm not sure if something like this can be practically implemented using existing methods.
Instead of manually reordering clauses, etc, you could just run the original essay through another LLM without watermarking capability and ask it to write a new essay based on the original.
Then test the result against your own plagiarism detector and iterate through the watermark-less LLM until the resulting essay passes.
Or just proactively run it through a bunch of times.
Or just use the watermark-less LLM to begin with.. personal, unshackled, powerful LLMs are definitely on the trajectory we're headed in.
I'm confused why you focus on plagiarism detection. That being said, your scenario is very briefly mentioned in the conclusion and requires augmenting the approach (entropy coding) with error correction.
The result would be that as long as your modifications (reordering clauses, etc.) reasonably closely follow a known distribution with limited entropy (which I think it clearly does, although specifying this distribution and dealing with the induced noisy channel might be very hard), there will be a way to do covert communication despite it, though probably only a very small amount of information can be transmitted reliably. For plagiarism detection, you only need a number of bits that scales like -log[your desired false positive rate] so it would seem theoretically possible. Theoretically it also doesn't matter if you use text or images, though in practice increasing the amount of transmitted data should make the task a lot easier. However, I'm not sure if something like this can be practically implemented using existing methods.