Earlier quoted context omitted.
Punishment always changes behavior only when you're looking/enforcing. You'll never know if you police kid enough. Is that your goal?
Replace your first word with "Reward" and re-read it. My own observation is that both of these claims are partly true but that there is some effect that remains even when they think they are not observed. Parents often see what kids think they don't, so I have empirical evidence of this (as all parents do).
Reward works well for building a healthy rapport and relationship. I don't think punishment is entirely ineffective, but if it isn't rare and judicious it may well end up causing more harm than help. The problem is that the response to punishment is visible immediately while the downsides may only appear many years later.