Of course it was a mistake. In fact, as of 20 minutes ago, the mistake appears to be sorted out, with both the main license file and the offending files sporting new copyright headers.

But corporations hiding behind their workers is a no-go. Corporations get to enjoy their successes, and it's fair to hold them accountable for their failures. Least Microsoft can do is a bit of public comms work detailing what they will do to ensure these mistakes are not repeated in the future.