As I have said in another comment, the authors did not just copy my short definition but my entire paragraph-length explanation of the concept. I might have let the short definition slide but an entire paragraph is just inexcusable.

I agree that this is not a school assignment, so the nature of plagiarism is a bit different. But you hit the nail right on the head. We should be worried that they are pretending to know something that they really do not. They copied my explanation nearly word-for-word. Doing that does not prove that they actually understand the concept. It only proves that they have copy and paste. Now maybe they did spend some time learning it and looking into, but there is no way to know for sure. The only way to prove that they really understand the concept fundamentally is to make them explain it themselves in their own words. And that is precisely what they should have done.