Correct. You have to keep the ratcheting state outside of Nostr, which means that either only one client had access to your DM and/or different clients see different DMs, or that you have a way to import and export the ratcheting state from app to app manually, off from nostr.

The later becomes a better point of attack. You don't need to break the decryption if you can just get the state by attacking the import/export function directly.