As I said, I am not a lawyer and you may have out-lawyered me here already, but I'll do my best to respond. The law prescribes this path for youtube-dl authors to respond to the claim, if youtube-dl authors want to put their names behind the project and make a legal case out of it. That severely limits the calculable damage that is possible, (especially if youtube-dl won't pursue the matter further.)
The claim in the takedown notice that is required to be submitted under penalty of perjury is simply that the party submitting the claim actually represents [copyright holder] and that notice which RIAA submitted also does not make any demonstrably false claims. It does not entirely fit the format of a regular DMCA copyright takedown request for copyright enforcement, it has two sections (one is called "Anticircumvention Violation"). It goes into detail about how the rights holders which RIAA lawyer represents are aggrieved, with language like:
> we have a good faith belief that most of the youtube-dl forks are infringing to the same extent as the parent repository.
# (This is probably the most dubious claim, and since the channel for takedown notices is for copyright enforcement, if your argument had a leg to stand on, I think it's this one. But is it calculable damage? And is the mention of Taylor Swift and other RIAA member artists in the README not plenty of evidence that there is actual infringement that is happening, or at least that it could have been asserted in good faith as it were that those rights holders believed there is a valid claim, as this infringement was happening?)
and
> the youtube-dl source code available on Github (which is the subject of this notice) circumvents YouTube’s rolling cipher to gain unauthorized access to copyrighted audio files, in violation of YouTube’s express terms of service
I think for this to be tortious interference, you would have to demonstrate that there was any intentionally false information in these claims, and that's going to be tough. There is part of a DMCA takedown claim that must be asserted under penalty of perjury, and after re-reading the law and jogging my memory I understand again that for the party sending the takedown notice, that is very limited. (Unlike the counter-claim, which has to assert ownership under penalty of perjury, the claim must only assert that claimant represents an owner as identified in the claim and that the factual claims made in the notice are true, in good faith.) Otherwise it's hard to argue that this notice is anything but an effort to enforce multiple sections of the law as it is written, by asking nicely for a hand through the channels that GitHub has made available for enforcement.
Whether or not it meets the definition of a valid DMCA takedown notice, it is a letter with many demonstrably true factual points, which GitHub has accepted through their channel for enforcement of claims. GitHub has "voluntarily" complied with their interpretation of the law here, in response, and there is an avenue for redress for the authors, if youtube-dl authors feel this is worth pursuing.