The point is that if a popular search engine returned pages primarily based on whether particular search terms appeared in their self-published indexes (rather than using a crawler that parsed the page content to build its own index), web sites could easily publish indexes that just listed popular terms users are known to search for regardless of whether the site had anything to do with those search terms. Pretty much what uses to happen 10-15 years ago when everyone believed meta tags were the way to achieve SEO.