He has been using HN over 900 days, and this is on the front page -- it's quite possible he simply saw the article and comments just like you and I did.
Sure it's possible. But, given that he hadn't commented or submitted on HN in 20+ days 'til now, it just seemed unlikely that he's currently actively delving into submissions/comments to the extent that he'd catch this so quickly.
So, I just found it not so much an "Aha! Busted!" moment, as much as it was noteworthy.
I did see this article, I just didn't read the comments. No idea that this would be here had it not been for that tweet.