Are these usages of "not" ellipsis or something else?
Banner Ads Are A Joke In The Real World, But( They are )Not In Class-Action Land.
Brad Pitt loves Marion Cotillard, ( he does ) not ( love ) Angelina Jolie.
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So obscure English time, "ellipsis" is singular, "ellipses" is plural. It's not a commonly used word so I wouldn't worry much about it.
Anyway to answer your question: Technically, yes, you are right, and these are elliptical sentences. Apparently in "formal" English you should only remove those parts of the sentence that are actually repetitive, and not just (the parts of the sentence that are) superfluous.
So "Brad Pitt loves Marion Cotillard, but (Brad Pitt does) not (love) Angelina Jolie" could be shortened by removing the stuff in parentheses. However the sentence "Brat Pitt loves Marion Cotillard, not Angelina Jolie" is not strictly formal, since you excluded the expected conjunction "but".
However, in this day and age it's not necessary to be formal. It's often more natural and native to write like a newspaper headline.