VS Code is a lot more than just a text editor; off the top of my head, it has:

* Syntax highlighting and code folding * Autocomplete * Integrated file browser * Integrated git client * Automatic formatting * File preview * Debugger * ...and a ton more features

Just looking at the screenshots from the release notes (https://code.visualstudio.com/updates/v1_36), I highly doubt this is "the very simplest cross-platform UI you could ever need to design".

Also, as far as I know, the vast majority of the editor is still HTML/CSS/JS, with only a few exceptions (the integrated terminal and project-wide search I think?)