It is fine to see “Followers only” posts because technically, it still is “public”, just limited to their followers. We can not make a sound decision if we can not see:
1. the entire thread 2. the context 3. all the other posts of the account
Another reason why it is “okay”, it is part of the moderator's / admin's task to, well, moderate. If there are content that they can not see, they can not do their tasks properly.
If they truly want something to be “private”, then they should use “Mentioned only” or better yet use an e2e chat platform.
The “Followers only” mode is meant for the regular users, not for moderators/admins. Otherwise, as it currently is, it is open for abuse. I wouldn't be surprised if spammers and scammers are already abusing this.