They allege that it circumvents a "rolling cipher", which (they say) is sufficiently DRMy that it comes under the law. The DMCA doesn't have much of a definition of what counts as an access control measure, but the "rolling cipher" that scrambles the download URL looks like one if you squint.
Exactly what counts as DRM is less of a technical question than a legal one, so (not being a copyright lawyer) it's not obvious to me what the right answer is.
https://github.com/github/dmca/blob/master/2020/10/2020-10-2...
There is legitimately some obfuscation happening:
https://www.jwz.org/blog/2013/06/youtube-download-counter-co...
