Well spotted! Just a few minor edits. But I do think it's important to get the message out - it's so easy to ignore, and then you can end up with real problems and real pain for a long time.
Both links are about posture thus making his comment relevant to both. If you mention something to someone concerning something you've done, do you never mention it again even with a different audience? The bigger question is "Why shouldn't he say the same thing?" It's not like he's spamming the front page with links. He has an experience that might be applicable to people who did not see the first link (like me, I come to HN multiple times a day but did not see the previous link or comment).
Pointing this out seems akin to hanging out with a friend in multiple social settings and when he tells a story you have heard before but the group hasn't, you stop him and tell him he's told that before.
Better that he repeats himself a lot then to mention it only once. It was the first time I saw this comment and as somebody who's developing aching wrists and fingers, I'm glad I came across it.