For more on how they found out:
https://octodon.soci...a/105085774214181683
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-wraith808
Who are "they"? And it seems kind of random for that person to accuse Google of doing the takedown. 
-Deozaan
They, being the group maintaining the repo.
How is it a surprise that the RIAA are taking down a streaming app they have not specifically allowed? They've been doing this for decades. And while I might dislike the RIAA, as a copyright holder, I'm all in favour of copyright holders' right to close unapproved access to copyrighted content.
-Dormouse
This is what they want you to think. The case for it is actually more nuanced.
The RIAA’s fraudulent attack on youtube-dl is not a DMCA §512 infringement/safe-harbour, and the reality is weirdTL;DR: This is not a 17 USC 512 infringement/safe-harbour, RIAA’s standing is highly questionable, it is threatening a member for an averred nonmember’s §1201 injury, any actual works duplication is not performed by youtube-dl’s developers directly, nor is the work itself or its test suite an infringement of RIAA / members copyrights, and numerous defences exist for routine use or incidental transmission or copies made by developers, hosting services,or others. Further, youtube-dl, digital and information liberties groups, Microsoft, and Google/Youtube should fight the RIAA’s claim.