I think they meant they are following the activity on this comment. More common on sites like reddit where commenting gives you notifications on a thread.
Agree that’s possible, though tried to think of a more valuable reasoning.
To me, posting comments with notes for yourself is basically spam, especially comment like “following” - which adds less value than just upvoting a comment; to follow comment, just click the timestamp to bookmark it and/or sign-in, upvote it, then look through your upvoted comments in your profile.
I think this is what 'favoriting' comments is for. Much easier to find than sifting through all comments ever upvoted. Your bookmark idea is also a good one.
Trying to find the best possible reading of a comment does not mean ignoring the literal meaning of a comment. If the comment was, as speculated, a reminder for the user, it’s not appropriate and counter to HN guidelines:
