Do you have a source you could share about admins only seeing the private messages of local users? That’s not my understanding.

Take a look at this post or this one . They say that the admins of both the sending and receiving instance could decide to read your direct messages.

Privacy isn’t just for illegal acts. (And plenty of laws are unjust) You’re right that for truly sensitive communication it’s better to choose a tool dedicated for that purpose. It can still be beneficial to add encrypted communication to direct messages.