I wasn't really saying anything about hndex, apart from that it indexes the contents of external articles submitted to HN, as you said.
The correction I was making was about HN Algolia search (which is linked from the search box at the bottom of each HN page), which only indexes content on news.ycombinator.com itself – i.e., article titles, comments and text-only posts like Show/Ask HN – but not the external content in submitted articles.
Most articles have no comments. Yet one can search on hndex for terms only found in the article, not in the title.