My perception was that the "retraction" was an attempt to reverse the effect of the original author's note. This is obviously not actually possible. While EY probably knows this, I think he is overestimating the actual benefit of the retraction (and of the related decision to suppress discussion derived from that information in these threads).

The people the retraction is most likely to [arguably] benefit are people who started reading after it was removed and people who were reading it at the time but were inattentive to the author's note and any discussion that happened in the intervening period. My assumption is that there are not actually very many people fitting that description participating in these threads. This is weighed against by the cost of imposing rot13 on all discussion derived from that information and arbitrary downvote penalties on people who are unaware of the rule (as well as acting as the spark that sets off arguments like this).

I also think that it's possible that HPMOR discussion would be better served by a conventional forum rather than the reddit engine, as some others have mentioned, and that this could mitigate the spoiler problem, but that's mostly unrelated.

He said "responding to unmarked edit" as though there was something wrong with failing to mark a simple addition made 10 seconds after the original post. I was confused, since it was not my experience that anyone considered this a problem anywhere.