"Readable by anybody with an admin account" is not the same as public. And as a bunch of people involved in January 6 found out, end-to-end-encrypting something doesn't keep mean it won't get revealed. So the general rule is assume anything you say online could be made public; use Signal (or some other encrypted messaging that you trust) and limit distribution to a small number of trusted people to reduce the chances of that happening -- but don't count on it!