No it doesn't. It makes it copied without authorization. Stolen means the original owner does not have access which is not the case[0]. This idea that copying is theft was propaganda invented by the MPA[1], and we ought to stop parroting it, even when it's Microsoft doing the unauthorized copying.
Plagiarism is theft because it does take something away from the original author (attribution). Plagiarism and piracy are different concepts. Making a copy and forking the code is not what they did wrong, that part was authorized. Deleting the author's name and pretending it was their original work is the issue.
The idea that piracy is theft was not invented by the MPAA. I arrived at that conclusion myself, and indeed most people I've interacted with find it to be pretty reasonable. It's only ever been a minority of giga-nerds who try to claim that "stealing" cannot cover situations involving a non-scarce resource.
When you download a movie from torrents, you don't submit it for an Oscar nomination claiming you've made it. You just copy a file to your computer intending to kill a few hours of your time while playing it back.
Microsoft®™, however, not only copied the code, but claimed it's theirs.
