Does it even matter if they know? The private content is supposed to be invisible to them anyway. The fact that they don’t know it exists would make it more invisible.
Also, some platforms, like Mastodon, have actually adopted some privacy. For example, they added “followers only” posts that only their followers can see. If they are aware of Mastodon’s “followers only” posts, then they already understand the basic concept of limited distribution.
Does it even matter if they know? The private content is supposed to be invisible to them anyway. The fact that they don’t know it exists would make it more invisible.
Also, some platforms, like Mastodon, have actually adopted some privacy. For example, they added “followers only” posts that only their followers can see. If they are aware of Mastodon’s “followers only” posts, then they already understand the basic concept of limited distribution.