"Plagiarism is the "wrongful appropriation" and "stealing and publication" of another author's "language, thoughts, ideas, or expressions" and the representation of them as one's own original work. Plagiarism is considered academic dishonesty and a breach of journalistic ethics."

Where is one claiming that the used "expression" is one's own work? Of course, nowhere.
Beyond that, a proper acknowledgement of every published record out there is not really feasible within a tweet itself.
If one really wants to push this into calling plagiarism, since the affected article is published under Creative Commons license, one should tell them that instead of using it for his/her own purpose in a forum - I guess, they will not move a finger, not only cause it is a tweet (and not a (scientific) article or patent) but also since that tweet is working toward the same goal as the article.