It's pretty common to submit a confidential draft. This started with the JOBS Act [1], which loosened disclosure requirements generally for "Emerging Growth Companies" [2] when they are going public and introduced the notion of a confidential draft registration statement. You originally had to meet certain requirements to qualify for confidential filing, but that privilege was subsequently expanded to all companies [3].
Exactly how common it is to announce your confidential filing is harder to assess, but it's not _that_ unusual. They likely decided their filing would leak and thus it was better to get ahead of it.