HN is more like a forum in that sense. I don't post for EVERYONE to see. I post for parent to see, at least primarily.
You can at least control who your posts are visible to on some social media platforms. Not so here. Your comments are public and after the edit window closes, permanent.
I feel like I can tightly control where my comment shows up. It will always be tied to the comment or post I'm responding to. Hard to take out of context etc. But you're right, there isn't something like FB groups that I can scope a message to.