Quick look .url saved webpages as rendered HTML
Quick Look doesn't make web requests, so it can't show the contents of the webpage without opening it. The question you linked was asking about .html files, with the actual source saved locally. .url files only include a URL, so without downloading the page it can't render anything. You can still use .html or .webarchive files and they will be rendered in Quick Look (with a .webarchive everything, including images and styles, will be included).