I think it's you who are confusing the semantic relationship between the two (three?) concepts.
Plagiarism is not a legal term; it refers to using someone else's intellectual work (which may or may not be under copyright), without credit, in a way that violates the cultural standards of an academic (or other) community.
Plagiarism may or may not be copyright infringement -- even if the materials plagiarized are under copyright, it may be fair use. Or in some cases you can plagiarize by stealing someone's ideas but not their text or other fixed expression (ideas are, in general, not protected by copyright; but stealing someone's ideas without attribution is still plagiarism in an academic context).
But the defenses against an accusation of copyright infringement, like fair use , do not serve as defenses against an accusation of plagiarism in an academic community. Plagiarism in and of itself is not illegal, there is no law against plagiarism. (Unless maybe in extreme cases it could be fraud?). Still, plagiarism may get you kicked out of school or denied tenure, or generally cause you to lose respect and trust. But some instances of plagiarism may, of course, also be illegal copyright infringement.
Copyright infringement is, of course, a legal term.
"Piracy", to me, in this context, is simply another term for copyright infringement. Isn't that how the large content owners use it? If it's copyright infringement, it's piracy.
Of course, whether an individual instance is copyright infringement or not, is decided for certain only in court. But there are cases reasonably educated observers would predict with high confidence as being determined (or not) to be copyright infringement; and others that might be considered a strong-but-not-certain case, etc.
Something can be both plagiarism and copyright infringement, or only one (either one), or neither. They are entirely separate.
In this case, I think it's likely fair use, and also perhaps not likely to be considered plagiarism (rather, isn't it documentary realism?). But the reason people get so excited to call it copyright infringement -- is because the movie studios get so excited to call copyright infringement uses of _their_ IP that reasonable people think ought to be obviously fair use too. "And since the producers of the movie consider it fair use to copy 14 of my slides without giving me credit, it must also be fair use to quote the scene of the movie here" The irony is entirely intended by the author of the OP.