Just spitballing, but maybe Tortuous Interference. The Youtube-DL developers and users have an advantageous business relationship with GitHub/Microsoft, which was interrupted and permanently harmed by the frivolous claim. Github has done this to mitigate the harm, but people will continue to question whether Microsoft will reliably serve, etc.
YouTube-DL could potentially argue Slander of Title, which is well established in terms of claiming ownership of another person's copyright. Claiming someone else's intellectual property is inherently illegal seems pretty similar.
It would be hard for YouTube-DL to prove damages, but with a showing of intent there could be room for punitive damages based on what the RIAA thought they stood to gain.
