I am not talking about this particular case, but about plagiarism in general.

In college, I remember the very same debate when we were told about plagiarism, and yes, copy/pasting your own work without giving proper credit, at least in the academic world, is considered plagiarism. This is due to the fact that how do we, the reader, know that it's your own work? All we see is two articles that are identical, or at least look very much alike, that are posted by two different people in two different places. The only person that knows for sure that it's the same person is the author. This is compounded by the fact that x amount of users who are on many different platforms often don't use the same name/icons, consequently, the article in one place seems to be stolen from someone else in another place. Hence the importance of telling people about your source material and making your identity in both places very clear and obvious.

The program I studied in used this website to help us when trying to cite works for our essays, it gives explanations as to how to cite, based on MLA standards. Now, don't get me wrong, I am not saying that academia is right on everything and that MLA citation is the only way to go, but we should all apply at least a base amount of citation when writing an article, essay, book or other.

Purdue OWL (Online Writing Labs): MLA formatting.
https://owl.english.purdue.edu/owl/resource/747/02/