alt Hacker NewsVideo decryption+decoding is a well-defined enough problem that you can ship silicon that does it. You can't do the same thing for the UI of a social media app.
You could put the entire app within TrustZone, but then you're not trusting the app vendor any less than you were before.
Although now I think about it more, you could have APIs for "decrypt this [text/image] with key $id, and render it as a secure overlay at coordinates ($x, $y)"