Yes it is plagiarism, but it’s perfectly legal.
Sometimes I will come across a case which I am not allowed to quote because it is deemed an “unpublished opinion.” If I think the arguments are germane to my case, I will literally steal the language of the opinion and put it in my brief without attribution.
Technically I could be sanctioned if I actually referenced where I stole the language from.
Per court rules you can’t reference an opinion that was not “certified for publication” (even though it is published) but you can literally steal the entire opinion and put in in your own brief as if they are your own words.
It’s stupid, but it’s the law.