I don't think the difference between Electron and Webui is the JavaScript thing you are talking about because Electron uses a bounded Chromium browser. Webui uses any installed web browser, so both are the same.
True differences:
1 - The big difference is that the Electron backend is ONLY JavaScript, while Webui can be used with any language, C, C++, JavaScript, Zig, Nim, V, Python, Go...
2 - Electron is big, yes, +100MB, but gives you complete control of the window, while Webui is 200Kb, but all it does is run a web browser for you and sends you click events, so you have no control over the window because the browser owns it, not your application. So, Webui is suitable only for quick software development, while "big" projects should use Electron.