I think it's just that search engine quality comes up often, and through the recent very popular Kagi threads, a lot of HN users have ended up at least giving it a try.

In a sense it's targeted advertising, since HN users are very likely to be power users who care about a paid search engine for one reason or another. But it's still organic in the sense that the articles posted are ultimately just the regular search engine discourse or otherwise not content specifically crafted for the purpose of advertisement.