Whatever anyone calls a "social media" is just as [non-]private as any other "social" place. What we publih is formed by the tool we use - and !gnusocial currently works as a very obviously public tool just as you mentioned.

I generally recommend private communication to be handled with some entirely different front-end. Be it XMPP (better) or SMTP (worse)... Just as long as it's not (intuitively) the same platform.