Is there anything I'm missing?
In person, a conversation is personal like this.
In this discussion forum, it's common to see a chain of replies in discussion/disagreement where each replier is a different person. -- The conversation is continuous in its content, not necessarily by who is replying.
People reading comments later will gain from a good comment/discussion, and may be able to offer a good reply in turn.
More people read than comment, so I'd encourage you to leave a good comment anyway.
https://news.ycombinator.com/threads?id=dang
(Of course the most common use is to see where you commented, but I rarely go beyond one page)
It will even include replies to your comments on that page.
React instead to the post in question or if reacting to a comment, write a reply for the whole HN community instead of just targeting one person.