Electron is a runtime environment that evolved out of a browser, not a browser. This app doesn't have any browser functionality I can see, like open arbitrary websites, but it does have its own system menus, icon, it's directly executable by the OS. By that logic no software written in an interpreted language can never be cross-platform, you'll always need a runtime environment.
Desktop files (on Linux & Windows) are just nothing than a text file with description/meta info and a link to local executable or any other sort of file* - that is NOT related in any way to executable at all.
> it's directly executable by the OS.
Because it is browser (Chromium) with a custom browser-extension by design.[0]
Without browser this "browser extension" (Web-app) could not run on any platform.