I believe you were referring to this case:
Project Veritas faces off in court with Democratic activist
If so, PV was not sued for defamation or libel, but rather for fraud (related to their operative's misrepresentation of herself to gain employment at the organization) and violation of wiretapping laws.
ACORN was primarily a defamation suit, but it was settled out of court, so you can't draw any legal conclusions from the outcome.
Actually that one's not ringing a bell either... I could be remembering the details from two different ones blended together. I distinctly remember there being some facet of the story where the "mark" thought he was on a date, got buzzed, and espoused a bunch of exaggerated and/or false info for the purposes of making himself sound important. And then that extraneous info was conveniently left out, and it was reported as "XYZ Corp employee says his company does this"
While it wasn't the particular avenue any of the aforementioned cases went down, it seems like with regards to this specific case against media matters, X may have to rely on the rather "untested" avenue that is "Libel by Omission"...to which, not many cases have been tried in that realm.
The Washington State Supreme Court ruled on one such case some years back (it was a case where a store owner had a mentally handicapped man thrown out of the store and charged with trespassing, and the local news affiliate reported on that true fact, while leaving out the fact that the homeless man had previously made physical threats toward the store owner's wife the last time he was in there...the end result was that it made him and his business look really bad)
The court said to meet the criteria, the plaintiff would have to effectively demonstrate two things
"
the communication left a false impression that would be contradicted by the inclusion of omitted facts.” (it met that criteria)
“
the language must not only be reasonably read to impart the false innuendo, but it must also affirmatively suggest that the author intends or endorses the inference.”
(the Washington State Case Failed the 2nd test because the plaintiff couldn't prove that the local news affiliate even had any knowledge of the extraneous past circumstances, therefore they couldn't prove intent of misleading on the part of the network as in order to intentionally omit for that purpose, they would've had to have known about it in the first place)
I think one could make a case that the Media Matters tactics could potentially both of those boxes.
"IF" the judge in this case uses a similar two-pronged standard.
The first one ... if people knew that MM created that outcome on purpose through algorithm manipulation, instead of believing it's just something that randomly happened, that could leave people with a very different impression of what happened.
For the second one, if Media Matters was going out of their way to take that false impression (that they manufactured), and provide that information to advertisers for the purposes of saying "you should stop advertising on X, look what we found", it would be hard to claim that the omission was inadvertent, and not for the explicit purposes of hurting X.