They wanted to reduce the duplicate work of maintaining multiple clients for each Platform, and created a rust library which can be used on all platforms.

And for the UI they use Jetpack Compose for the Android App and SwiftUI (I think that's how its called, correct me if I'm wrong) for iOS.

This way only the UI is "duplicate" but the core logic is maintained in the rust library.

EDIT: Okay just found my comment from 300 days ago: https://news.ycombinator.com/item?id=38402710

Element X is a totally new implementation, done in Rust. It's fast and nice but for now some features are missing compared to the old one.

Including critical ones like threading...

it's a rewrite. it'll replace the old app once it has full feature parity with it.