It wasn’t that video. At the Media Matters site they have a description of the leaked videos. There are 4 articles about their contents. Which makes this C and D letter weird, or at least intended for some other legal proceeding down the road.
C and D letters have two targets: either some small entity who blindly swiped IP and doesn’t have representation or an organization which deals all the time with first amendment issues and has legal representation. MMFA doesn’t need to broadcast the videos, they can publish a description of their content and hold the videos for proof of their veracity. Fox can’t suppress the descriptions and they know it. So the C and D letter has to be about something else.
They claim that the video obtained by Media Matters that included Tucker Carlson's infamous "that's not how white men fight" comment was proprietary IP and demanding that they pull the videos. MMFA's response from chief Angelo Carusone: “Reporting on newsworthy leaked material is a cornerstone of journalism. For Fox to argue otherwise is absurd and further dispels any pretense that they’re a news organization...Perhaps if I tell them that the footage came from a combination of WikiLeaks and Hunter Biden’s laptop, it will alleviate their concerns.":