> (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. This was conspicuously absent from all discussions I've read.)
It's absent because RIAA's intent is not stated. They did a blanket takedown, unprompted. As far as I can tell they never requested any particular modification. IIRC the only hint that it might be related is a mention that the rolling cipher algorithm that YouTube-dl "circumvents" was ruled to be DRM under German law.
However, the bulk of the DMCA seems to be leveled at the marketing of ytdl as a circumvention tool, citing unit tests containing metadata referencing RIAA-owned content (unit tests, apparently, are now part of 'marketing,' I guess.)