The article mentions that new Outlook uses a Webview2 control as the UI. You can call it JS/Webview.

Similar to Electron, but using an MS Edge browser window control as the webview control instead of Google Chrome.

It's lighter weight than Electron, if you consider that Webview2 ships with Windows and is shared with other Webview2 consumers as opposed to Electron apps which each ship with their own self-contained web browser.