Observation: this is a time sensitive post by the forum's owner/maintainer/main moderator on the original topic of the forum. Yet, he did not need to rely on a "sticky" thread or any other kind of special modifier.
It would actually be more interesting to me if pg had some quantitative way of knowing that this post would reach the top (some function of the # of YC applicants now nervously refreshing), but if I had to guess, I'd say he just knew from experience or something.