According to the first article, the code was hosted with a custom version of Gitlab, with the register link still functioning. Once an account was created all the repos were public. If that's true, then it's a public site being accessed through features of the site.
I'm sure it also depends on if the site was intended to be accessed "publicly" or not. Let's say, visually, all registration links were removed, but (as someone with internal knowledge of GitLab here did) could "breach" into the registration page.
