A good well-actually here often adds a lot to the discussion: it brings up points that others may have forgotten, elaborates on concepts that can be very complex, points the anonymous reader toward other resources that they may never have heard of, and can lend validation and social proof to ideas. And the negatives are curtailed by the short, high-population, transactional nature of message boards. When someone corrects you on Hacker News, it doesn't make you feel stupid (I hope): it just means that someone out there, among the hundreds of thousands of readers, had better information than you did. There's no real jockeying for social status on Hacker News, because if you get karma or score that point - who cares? You never see them again.
That's very different from a school environment, where you are pushing your comfort zone in front of a small number of classmates, and a correction can make you feel stupid and negatively impact your confidence. You have to learn with these people for a while; if you feel devalued from the beginning, your learning is going to suffer.
I can understand "This is especially annoying when the correction has no bearing on the actual conversation." case in a face-to-face conversation but the manual forbids all corrections whether they are essential to the discussion or not. Or should the rule be interpreted as: essential corrections are allowed but the literal phrase "well, actually" is forbidden?
If someone says if X then Y; you should be allowed to say: actually X' and therefore Z i.e., a subtle imprecision in the premise might lead to the big difference in outcomes.
Otherwise technical discussion degrades to a smalltalk.