Usually "no" adds nothing to the discussion. However, in this context I read it as "Obviously, no. The post author clearly hasn't put much thought on what they

Usually "no" adds nothing to the discussion. However, in this context I read it as "Obviously, no. The post author clearly hasn't put much thought on what they wrote, so let's not even bother dissecting it". In a way, it is a constructive comment in the sense that it prevents fruitless discussion. Of course one could just flag the post but that isn't as visible.