1. The proper question is "When a commenter is very wrong on a technical topic, is it net-positive for someone with expertise to point this out even if they don't contribute further?". I think the answer is obviously "yes". You seem to be more concerned with whether people are pulling their fair share, vs. being exempt/special, which I claim is the wrong question.

2. Effort scales with detail. The example comment you give is 3 times the effort, and I wouldn't have made the original comment if it required that much detail. My comment took me a few seconds to write, and merely tracking down a URL link for explanation would have made it not worth it.

3. Rebutting the N-th confused post on HN about dark matter doesn't interest me, but the meta point of what contributes to a discussion does interest me.