Maybe he's using them as a marker, as in which posts he's already read?
I know there's the bell icon at the top, but sometimes you misclick out of the list and then it sets every message to 'read' and you have no clue which threads actually have posts you haven't seen yet. So all things being equal, i think the laughing emote was picked due to it being relatively neutral and overlooked.
Or i'm wrong, which is also possible, but on a game forum focusing mostly on games in development, what's a little extra theorycrafting here and there?