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.
@Comment105 Commercial interests and propaganda will come to the fediverse. The propaganda is already here if you know where to look.
The biggest strength of the fediverse is that administrators and users have control over what they consume. They are not force-fed information from third-parties without their consent.
So, companies and politicians are going to show up on the fediverse. But you don’t have to follow them.