Any good community discussion site that maintains for years needs various forms of "manipulation", and this is a pretty minor one.
Adding an * footnoting what was done wouldn't serve much purpose.
Edit: Semi-related issue for comparison: one time, a contact DMed me on Discord to ask where a certain event was going to be held. Before I saw the message, she found the answer through other means and thus "helpfully" deleted her message to me.
Same problem: that actually makes it more confusing because I see inconsistent state: the notification lives on in my phone, but it's no longer present in the chat history. Better to just say "never mind" than to modify history.
but I suppose HN likes to keep things simple (even if it's a bit confusing).