Rust is hardly a "desktop-native" language. I expect it will have to jump through all sorts of hoops to interface with the Obj-C underpinnings of MacOS desktop libraries.

Regardless, even if the backend were pure Obj-C, the point is still that now you have a UI stack and a backend stack, instead of a single desktop-native stack, and you have to manage their communications. But I guess the money they save by not managing desktop-specific codebases eventually adds up...