Honestly the only time I even really notice usernames is when I find one person who is replying to a lot of threads in a single post.

Often I notice it because they are repeating the same rhetoric over and over, and it's usually something I strongly disagree with.

Even in those cases, I haven't internalized the names enough to recognize them in other threads.

I think twice I've read a post and thought oh hey that sounds like X. Mostly because of some niche knowledge that I've seen on here before and possibly interacted with. It's possible I'm strange though, I rarely read authors of articles unless they seem familiar or are worth recommending.

The only time I look at usernames is when looking at thread replies to see if the person replying is the same.

A recent comment by dang mentioned that some users request specific usernames that are already taken (https://news.ycombinator.com/item?id=32133471 ), and there was a bit of pushback and confusion about it.

I'd agree, the impact is minimal, but if you're an avid reader of the site, it's easy to start recognizing usernames if you're that sort of person. Situational for sure.