I agree that it shouldn't, but for whatever reason (possibly a limitation of HN's underlying software, or for algorithmic/engagement reasons) it does.

It does so that the ranking algorithm still works appropriately, and so that you can still reply, which otherwise you can’t for a day-old comment.

Yea that’s what I found surprising: the re-timestamping of the article and the comments.

The system is described here: https://news.ycombinator.com/item?id=26998308 & https://news.ycombinator.com/item?id=25061616