The complaint for declaratory judgment may be wrong for some reason--we'll know once the court decides--but it isn't wrong for the reason you suggest.
If you look at the document, it's clear that it's seeking a declaration regarding copyright within the United States only. (It could hardly do otherwise, since the court has no jurisdiction in Mexico or any other non-U.S. territory.)
If the term of copyright is different in Mexico, that's certainly interesting, but it doesn't affect this case.
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