Yes, this is only an issue for cross domain prefetch. With HTML resources, the goal of prefetch is typically not to get a head start on loading enormous amount

Yes, this is only an issue for cross domain prefetch. With HTML resources, the goal of prefetch is typically not to get a head start on loading enormous amounts of data, but instead to knock a link off of the critical path. The HTML typically references many different resources (JS, CSS, images, etc) and, if the HTML was successfully prefetched, when the browser starts trying to load the page for real it then can kick off the requests for those resources immediately.