They mentioned elsewhere it started off as a fork of the Xi editor which is written in Rust. Xi separates the text-editor idea into backend and frontend, and Xi itself is primarily the Rust-based backend which concerns itself with making editing operations very fast, and then you can write a front-end client for whatever language/platform you want.

In this case, they can use a fast Rust backend for multiple platforms (or just one) and then use Swift to make a nice, native front-end for MacOS and other native UI languages for other platforms if they wish.

Cross platform future aside, they want to use all the good libs Rust has for ropes and the like. The code is based on another editor earlier work, and that's in Rust.

They also know Rust so if they want to use Rust, what's the problem in using it? They don't seem to have any problem with building a UI on top, it's built and looks/works fine.