Its my understanding WebView uses GTK. for ios/kde I don't use WebView
Web UI is not native UI, even if it's technically embedded in a GTK/etc app. Native UI would be using the platform's native controls.
This project looks more similar to Tauri[0] than anything related to native UI.
[0]: https://tauri.app/
But that doesn't make your library native at all — just as Tauri is not native, neither is Sans, and that is the point being made here.
It's not native.