Each YC funded startup gets one "Launch HN" post (see https://news.ycombinator.com/newsfaq.html, https://news.ycombinator.com/launches, https://news.ycombinator.com/yli.html). It's fine if they've already launched. If they're already well-known to the community it might be different, but that's rare. In this case, for example, none of the posts you linked to got much attention.