Then the instance admin holds the private key and can still decrypt.
If you cared that much about privacy in DMs, we should have a “profile page”. Post a PGP public key there. Then you can send PGP encrypted messages to anyone who you have a public key for.
People go with the default “name brand” unless they have reason to think otherwise.
You search “Reddit” on the App Store, naturally most people are going to pick “Reddit” by “Reddit”. Not Apollo, or boost, or BaconReader, or RIF, or any other (absolutely amazing) third party app, unless they have some reason to think that it’s better than the official app. Which they don’t, unless someone told them otherwise, because they are conditioned to the cluttered ad-ridden garbage interface of new Reddit, and most people don’t need mod tools or accessibility features.