It’s not just “opening a website in a browser”. The UI itself is displayed in a WebView, but Electron (and Tauri etc.) also have code running outside the WebView which interfaces with native APIs etc. Just for a simple example - a normal browser application couldn’t open a file on your drive before without showing a dialog (and it still can’t without getting permission first), which Electron etc. can.
And this code is absolutely cross-platform. So how is it rich to call an application with a cross-platform UI and cross-platform functionality “cross-platform”?