I meant it's a small niche for Flutter to address only the apps that want to be cross-platform, don't want to invest into having "native" iOS and Android versions, but are Ok with using a different technology for desktop/web (or don't need those).
Either go mobile Web, or native.
They use Jetpack on Android
https://github.com/vector-im/element-x-android
And SwiftUI on iOS
https://github.com/vector-im/element-x-ios
But both use the same underlying Matrix Rust SDK
https://github.com/matrix-org/matrix-rust-sdk
So they share the core part of the app between platforms, but everything user facing is native