> it's just a personal attack

pointing out someone's stake in an argument does not constitute an ad hominem fallacy

It's not an ok thing to do here, regardless where it fits (or not) in the list of logical fallacies. There's a whole section in the site's guidelines about this under 'In comments'. The comment in question flunks just about all of these.

https://news.ycombinator.com/newsguidelines.html

Still doesn't add anything useful to the conversation

It literally does. Whether someone has a stake in an argument does not affect whether it is true or false.