It is, if you have any interest in convincing him otherwise, and successfully holding a conversation. If your goal is to argue aimlessly, and simply make your anti-claim with no new information (thereby convincing no one), then yes, you have no further tasks to be executed. But then you might as well have not spoken in the first place.

Otherwise, all you’re claiming afaict is that the posters should argue with themselves — find the counter-evidence to their own claims, which is likely sufficiently convincing that they would not have had any reason to speak in the first place.

That is, this idea of “not anyone’s job to search for (counter)evidence” is antagonistic to any form of discussion where parties disagree. If you want to make the opposing claim successfully, its quite useful to have evidence to support it (it’s not their job to find it for you... its not anyone’s job to do anything in an HN thread.)