Youtube-dl will not win this argument based on technical details on how content is provided by a licensee to an end user. That's irrelevant to how copyright law works. The entire history of copyright law has been about preventing the unauthorized copying of protected works through technical means, so copyright laws are deliberately agnostic to how the technical details are implemented. In the case of the DMCA, all that is required is that some technical effort went into restricting the unauthorized copying of licensed content.
A lot of HN commenters have pointed out that the technical efforts were quite simple: obfuscating the name of the cache file in the browser's cache folder. But for legal purposes that is sufficient. And if it's not, then in the very near future Youtube will no longer store RIAA music videos (or possibly any content) in an accessible cache folder, and congratulations: youtube-dl (and its competitors) won't work at all for it's namesake site.