@tokyo_0 Yep. A private account or a followers-only post should be seen if they reply to someone else's post or tag them.
If I'm not mistaken, that's how it works in Facebook. If you tag someone in a friends or followers only post, they can see and reply to that thread (and everyone else who can see that thread). (They're automatically added to the dropdown related to who can view the post.)
Hmm, I think you're right, Mastodon's tagging is basically DM. I sometimes tag someone in a followers-only post, and some of them replies.