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.