There were no such links in the documentation. Rather, various youtube partners like VEVO have special encoding schemes and the youtube-dl test suite includes tests for handling these schemes. Because the different styles are only used on partner content it isn't possible for those specific test case to use a CC-BY(-SA) work. Moreover, the test cases only download under 1 second of video and don't even store it.

AFAICT all the documentation examples are test videos uploaded by the youtube-dl authors themselves and ones by the Linux Foundation.

The complaint submitted to github was not a DMCA takedown (17 U.S. Code § 512), though it was superficially formatted like one. It was a complaint regarding 17 U.S. Code § 1201 anti-circumvention. There is no counter-claim (or takedown, for that matter) provision in that law because there is no safe harbor.

Moreover, your description of a §512 takedown is simply incorrect. Github is not obligated to take down a work when they receive a well formed DMCA notice: rather, they obtain an immunity from lawsuit for that particular infringement if they follow the procedure. In cases where there is transparently no infringement that immunity isn't worth much. In all cases it's simply a business decision to accept the liability or not vs the interruption in customer relationships, distrust, and loss of goodwill that can result from cutting off a user... no different than the decision to take on liability in any other business activity.

Maybe try avoiding such strong language when you haven't personally researched the subject in great detail?