Highly Cited: the article that appears to be most frequently. referenced by other articles in this story.
The Google News Support article also notes a few features apparently related to sentiment analysis (opinion, satire, in-depth), and those do ring a bell, making me believe we'd discussed that here*. I'm not sure whether "opinion", eg, is so identified because Google's analyzed it by content, or simply because it's labeled as opinion within the news publication.
* Google "News" is a hard topic to search for in this forum because of the name of the forum.
Just playing devils advocate... maybe they feel that "most cited" is too broad a claim, since they also say "appears to be most frequently referenced", and their sample size isn't necessarily all encompassing.