We use could for general ability. However, when you want to say that somebody did something in a specific situation, use was/ were be able to or managed to (not could).
There was a boy in the river but people were able to save him.
This is not an ability, but a specific event.
However, negative 'couldn't' is possible in all situations.
I couldn't swim.
We couldn't save the kid.
He played well, but he couldn't beat me.
Lastly, I will give you an example for you to think about it:
Jack was an excellent player. He could beat anybody. (He had the general ability to beat anybody)
Ted and Jack played tennis yesterday. Jack played great but Ted was able to beat him. (Ted, in this specific event, managed to beat Jack)
I can understand the person who suggested 'repeated actions'. When it is a general ability, you will repeat it probably.