The instructions being given there are describing a really, really bad way of going about that. To be specific, the changes they're recommending will make it so that, when a user requests a page on your site that does not exist, the script will start up a second request to your site to get the contents of your home page, then paste the results into the page that had the error. Among other issues, this will end up dramatically increasing the load on your server if something is going on that is causing a lot of 404 errors. It may also cause some confusion for search engines, since all of these "404" pages will now have what appears to be real content on them.
I'm not quite familiar enough with Joomla to give exact directions for how to do this properly, but the correct way to go about this would be to make your error pages actually redirect to the home page, not just include the home page on them.