Yeah no, automated DMCA notices should never be a thing. The shoot first ask questions later mechanic of DMCA notices is bad enough but at least have the decency to verify the claims you are making.

As lame as it is, it's not really plagiarism when there is attribution, even if it is non obviously that URL at the top of the page.

Attribution does not exempt an author from the need to get permission. Baring a few edge cases, such as fair use, a copying without permission is still a copyright violation, attribution or not.

Plagiarism is not the same as copyright infringement.

"Plagiarism" is not a legal thing: it's not a crime codified in law and doesn't appear in laws [1]. Instead, usually it's a violation of some internal code, e.g. a school's or newspaper's policy of integrity. As you say indeed, usually these policies only consider something plagiarism if there is a lack of (clear) attribution.

However copyright is encoded in law and copyright infringement is a crime. The linked page clearly violates the copyright of OP.

[1] https://en.wikipedia.org/wiki/Plagiarism#Legal_aspects