How does one sync from slack but hide messages from unauthenticated viewers?

You can mark something as private but you'd have to invite the users to join the channel. You might want a private community which makes the entire community hidden unless you have a password. It is a feature we have on the premium plan. We're working out a new pricing plan soon for smaller communities. If you are interested you can upgrade to premium or I can reach out over email and we can work something out.