They hadn't finished playing by 18.00.
They didn't finish playing by 18.00.
I have my own explanation what the difference is between past perfect and simple past when we use the tenses in such context. Could you please agree or disagree with it with your argumentation?
Past perfect helps us to say that 'they' have not finished their game yet and we need to wait (for example) if we want to speak with the players. Past simple doesn't give us any information about the future actions. We don't know if the game was going after 18.00 or not. For example, the game was interrupted with an important phone call or their free time was over. Right?