On HN the cutoff date for reposts is about a year. This is in the FAQ: https://news.ycombinator.com/newsfaq.html. Btw, it was part of how pg trained me to moderate HN back in 2012, so I assume it was always the policy.
My apologies. I didn't realize there was an explicit policy.
I don't think he posts the links because it's a duplicate post. He posts them when there is interesting and relevant conversation from that last time it was posted.
Edit: I see now that the post itself is tagged with [dupe]