IANAL but I don't even think DMCA applies. It's not like YouTube-dl is breaking copy protection schemes, at most it is handling anti-abuse schemes. After all, it's not like you can go and bypass Widevine with youtube-dl.
Unfortunately, it is making a copy. These guys also fucked up by "marketing" it for use in circumventing such controls. They should have showed how to pull OCW lectures.
I can understand why it would seem like circumvention, but one usage of YouTube-dl even with the README invocations is so that you can listen to or watch the media in a personal capacity. Not technically any different from how you might using the actual YouTube client, albeit probably in violation of YouTube's ToS. "Copying" in a legal sense might need a little more rigor. (IANAL either, though, so hell if I know.)
