[SahAssar]

That's not because of JS or WASM, that's because of the browser. Using JS or WASM does not mean you need to use a browser, you can for example run quickjs, node or whatever other runtime without bundling a browser.

This editor could have been written in js and use the exact same libs for rendering. If it used one of the smaller js runtimes (like quickjs) I'm guessing it would have been similar in size and pretty close in performance.

I don't get why you are conflating using a certain language with bundling a full browser (like electron does). It's two completely different things.