Because I've received Content ID notices before. The wording is very different. First you "dispute" the Content ID notice, and then the appeal is rejected, and

Because I've received Content ID notices before. The wording is very different. First you "dispute" the Content ID notice, and then the appeal is rejected, and it gives you a separate form to send a "counter-notice", which contains explicit warnings that the copyright holder can then sue you after you send the notice. Sure it's an assumption, but based on the reading of this article, it seems very clear that this was the result of a DMCA notice.