The code is also compiled to native code, but it's written declaratively. It's not for everyone, but it's not the same as Electron. Even still you have the option to use the traditional imperative API.
(Sourcegraph dev here) Just to be clear, the Sourcegraph Editor is built on top of Visual Studio Code, not just Monaco. Although we did use just Monaco on the website (not editor) in the past. :)
They try, but there is not really predictable and simple way to select those fonts in CSS. I've had times when Courier was used on Linux because the MS fonts we installed.
I would prefer they just do font-family: monospace