Probably a way for him to "Save" the comment. Typing "Thank You!" would have been more effective and less awkward.

Hacker News allows anyone with an account to favorite a comment, no need to comment on it to save it. Bookmarks are also a thing for a while.

In this context, an exclamation mark by itself stands for feelings of surprise and importance.

It is purposefully not a like or upvote or a bookmark – it is a declaration of importance of the parent comment.