Obviously the license was broken by removing the copyright notice of the original author, but it also doesn't sound like the author really did anything to try and handle the issue properly so now they are posting a rant.
Here is the extent of what the author did, "So I reached out to people I knew at Microsoft. This was probably a year ago now. They were shocked and apologized. But since then nothing has happened."
We don't know who those people are, what they're roles are, but I suspect they are not involved with the Rush project nor is it their job to handle potential copyright violations for all of Microsoft.
The author could have posted an issue to the repo expressing their concern. If this was ignored then they could issue a DMCA takedown notice through GitHub. If this was not successful then the author could contact Microsoft legal or file a lawsuit.