Ah, the infamous socks question.
Yes, the "reddited" notice is one of the notices mentioned in the question you linked to, and it serves the same purpose as the other notices:
Post notices are short text boxes that are attached to posts in order to annotate some exceptional situation. They allow ♦ moderators or bounty donors to let other users know something about the post outside of the normal channels of edits, votes, comments and answers.
It's only available on Stack Overflow, and I'm not sure if it's available to moderators, I think it's employee-only. And as you've already guessed, the exceptional situation the notice addresses is that the question hit the front page of reddit/r/programming or other similarly high traffic link sharing sites (I'm looking at you, Hacker News), and as a result it has attracted a lot of visitors from outside StackVerse.
As you probably already noticed, the question generated a couple (or more) close / re-open wars. I don't know why Shog9 applied the notice when he did, but to me it read a bit like:
Hey all, lots of eyes on the question right now, can we please stop changing its state every few minutes? We're starting to appear a bit foolish to outsiders. Kthnxbai!
Also, there were a lot of off topic comments, both on the question and its answers, and more than a couple joke answers. Meta commentary, side discussions, good jokes, bad jokes, etc, get tiring after a while.