Agreed. They’re using that argument to frame their breach as a win. The reality is that open source is easier to reverse engineer and find vulnerabilities in because you have the source. Our researchers do this every day and closed source makes that harder. Advocacy debates in favor of open source have muddied this conversation - but that is the cold hard reality.

Now that an adversary has MS’s source code, it is indeed easier for them to do vulnerability research. So this is a net loss for MSs overall security posture, not a win.