Outside the scope of the paper, but sure, it's a valid and interesting question.

If a model were to produce a pre-existing image not in its dataset, then we can strongly suspect that two behaviours have occurred (for the sake of establishing provenance).

First, it was just the ever so small, non-zero chance that it was produced by a random process that was not based on anything learnt, and within the finite resolution and some accuracy of colour, happens to match up to something already existing which, for example, may have been produced independently by a human artist - say, after the dataset was collected, but before the model was trained, such that it is emphatically a "coincidence" and nothing else, no collusion, no learning, no intelligence. Just chance.

Second, and I believe far more likely, this image has been encoded into the model by indirect learning and proxy. For example, say some notable and famous work of art is not part of your dataset (which is believable). However, say that there exist, say, art that references this famous work in some way (as artists sometimes do), which may be up to the point of parody (or some a gag), or may be some small aspect (colour scheme, notable style, outfits, lighting & composition, certain people, etc). Especially if it is what we would literally call "influential", is it not possible to reconstruct the famous work (that was not in the dataset) by indirectly learning about it from other pieces? Now, exact (or near-exact) matches are unlikely, for the most part it would probably tend more towards similarly repeating the references and parody of its dataset, but I feel this would strongly increase the likelihood of randomly producing it as in the first behaviour, and we're inflating the chances certain works are reconstructed entirely by "coincidence"; at least, as according to all observers that merely look at the binary presence of the original in the dataset.

Is this a copy? Is it not a copy? In all honesty, the notion of copy or not is insufficiently defined, because there's always some angle that can be part of it, or may not be, as most copyright law is handled on a case by case basis. Is it a copy because its provenance was an attempt to copy? Was it a copy by cosmic accident or just an independent work? Was it a copy by inferred reconstruction, thus not literal copying from the source, even if a verbatim reproduction is made? Is a vector version of a raster image a copy, and vice versa? What is "sufficiently" transformative to no longer be a copy? Does a copy of the Mona Lisa exist in the mind of a person that experiences it, while as a subject in a photograph it has grounds to not be? I'm trying to avoid anthropomorphising it and state whichever way what this is, I'd rather just illustrate how what we learn, what we experience, what we produce replicas of, and all of this is generally part of an unsolved problem/field: epistemology. Do we understand diffusion models fully? No. Therefore, weighing on one side or another should merely be for the purpose of a leading research question, or to exist on either side of a dialectic; such as in the courtroom. Perhaps we're still just too immature on this topic to really say anything about copying.

Really, the only thing we do know about copying, is that you should never get caught doing it.
(For legal reasons, that was a joke.)