> The users in question are posting on a public forum under a username of their choosing, and it's still an invasion of privacy to read those posts and play pretend matchmaker?

Yes it is. There is an aggregation happening, that is not available on Hacker News per se. To make this aggregation easily available to anyone else is some kind of privacy invasion. To make matters worse, usernames can be traced back to IRL individuals, depending on the information they put into their "About" page on Hacker News. So yes, this is indeed an issue.