Not a lawyer, software guy working in licensing & copyright for 20+ years. Was paid for a few years to talk about this stuff with tech and publishing people.
They're requesting the takedown under 17 U.S. Code § 1201 - Circumvention of copyright protection systems. [https://www.law.cornell.edu/uscode/text/17/1201]
It's a few paragraphs of the actual law at stake here. There's no much to it.
Essentially, they're arguing that Youtube's normal stream distribution technology is "effectively control[ling] access to a work." Given 3A & B from the link above, that will take a fair amount of arguing - there's no encryption, there's nothing that requires information under the authority of the copyright holder (like a key) to descramble the information.
Unless the repo has code that's breaking browser-based DRM, in which all bets are off - breaking DRM is by definition circumventing a technical protection. Doesn't matter if it was easy to break-- you break encryption, there's no more argument over whether you're circumventing. Decryption is right there in the text of the law.
Github agreed to the takedown because they don't want to be distributing a circumvention tool, and they don't feel like going to court over whether this is a circumvention tool or not. There are a lot of repos out there, you can't go to court over every single one. I'd be surprised if it stayed up on GitLab for much more than a New York minute, either.
The EFF might fight this, because there's a pretty good argument around the noninfringing uses; however, they also might not, because there's not much of an argument around whether it violates the Google Terms.
Some us are mentioning that there are use cases for this software that don't infringe copyright. That goes back to Sony v Universal, the VCR/Betamax case, which permitted the production and sale of technologies with "substantial noninfringing uses." In the case of the VCR, the particular noninfringing use was time-shifting of broadcast television, taping shows to watch later. The noninfringing uses here are around downloading works that are in the public domain or Creative Commons, and definitely around offline use. Easily a colorable argument.
The file sharing cases of the early 2000's have drawn some lines around the noninfringing use defense, though. Napster and Grokster both claimed, in court, to support noninfringing uses. However, both products promoted their products as offering free access to copyright-protected works, and the courts took notice of that in both cases-- I believe that the Grokster opinion may have noted that Grokster had never mentioned a noninfringing use outside the trial.
IOW, if you're providing a dual-use technology for noninfringing use that is also capable of infringing use, you absolutely cannot promote the infringing uses to the exclusion of others. Not only do you draw unwanted attention to yourself, you may actually (as in Napster and Grokster) invalidate the most important defense that you have for your activities.