My (flawed?) understanding of DMCA is that the project whom a DMCA is filed against can counter-file if they believe the DMCA is illegitimate, after which the burden of going to court is between the group that filed the DMCA and the group they targeted.
The problem with both RIAA and Google takedown demands is never about the DMCA takedown in relation to copyright infringement, its the circumvention (it seems that EFF understand this very well, but most online commentors didn't get this, which was exarcabated by GitHub using the DMCA takedown repo). Now, no one knows if GitHub were also directly targeted by legal threats as an acessory to "enable" the distribution of circumvention tools, which RIAA and Google is arguing.
Also, what RIAA is trying to remove is the code that allows to get the music video files from YouTube, which is served differently to normal videos (not just the test units in question). This was conspicuously absent from all discussions I've read.
Someone else in this thread linked this, which seems like a useful resource as well: https://sourcehut.org/blog/2020-10-29-how-mailing-lists-prev...
Edit: It was biryani_chicken who linked the resource above.
Also, I misunderstood about the legitimacy of this specific claim, my brain had yet to shift gear from the youtube-dl shenanigans.
Thanks for the response!