From what I know, DMCA notice requires the identification of claimed copyrighted content and this document does no such thing.
microsoft isn't gonna fight the good fight. There's no reward (to them) for doing so, and there's only a cost. It'd be a stupid business decision.
What is more correct is to lobby legislation changes to copyright. Repeal the DMCA, or change copyright law to such that you cannot abuse it this way. And this lobbying is the civil responsibility of everyone, not just microsoft.
Stupidity has no limits.
For more info: https://en.m.wikipedia.org/wiki/Censorship_of_GitHub
"It's necessary that the entirety of GitHub be shut down until this case is decided. As noted here, they have persistently failed to adhere to our DMCA take-down notices, and continue to knowingly host content that circumvents our copyright protection schemes. Furthermore, it is not sufficient to just remove this content, because it has been and will be repeatedly readded, and GitHub has taken no action to remove these extra copies. We are likely to succeed on our merits because <blah>. We are irreparably harmed because <blah>."
That doesn't mean they'd necessarily win on a broad injunction, but they can still push for one (see Apple v Epic for a recent example). And stunts like this one make it more likely that a judge might agree that a broad injunction is necessary instead of a narrow one.
GitHub may be the single source of truth, but it’s not the only copy of the data.
Otherwise they're getting GitHub's best effort, which is exactly what they've paid for.
If MS didn't comply with the DMCA takedown notice, then the RIAA could go to court and get an injunction to force it down. Depending on how spiteful the RIAA and the judge are feeling, the injunction could be worded to take down the entirety of GitHub over a single repository. And the impact of actions like these is to make it more likely that such an expansive injunction is sought or granted.