That’s my point, I think. It seems to me that moderators are doing the right thing by HN rules and context by tagging such discussions are not off-topic. Why is it described as “unfortunate” then?

I think you're reading it as "Unfortunately, moderators occasionally unkill such threads..." when it's meant as "...they tend to be flagged to death by users regardless, unfortunately".

I think so too perhaps and hopefully, but there is no comma As in your example. Just a period suggesting the previous sentence has ended.

True. But the following phrase is set off by both a period and a set of parentheses. I think it’s pretty clear the author’s intent is for there to be a complete sentence about the behavior of HN users and then a complete thought saying that was unfortunate. (Then a parenthetical about the moderator behavior.)