The detection system does work — it fired for me as recently as this week — but it's not instantaneous. It only fires after someone else saves, and even then I believe there's something like 30 seconds of polling delay.
In this case, it seems that a bunch of you noticed the same poorly formatted code almost as soon as the question was posted, and you all jumped on the easy cleanup edit; note how similar all the edits are. Adding four spaces is probably so natural to you all (rep of 3k, 61k, 22k, 30k, 66k) that you all submitted your edits and clobbered each other before the poor detector had a chance to figure out what was going on and send the notification out.
