Indeed - that's what their policy
broadly states. I presume in this case we're talking about a local authority of some sort - there is an historical custom and practice for destroying clerk's notes once the minute of the meeting is agreed, and even if that wasn't documented I'm not surprised the ICO decided not to take action.
That's not to say that the ICO necessarily agreed with the public authority about destroying the notes. They might even have thought it whiffed a bit, but they have to bring prosecutions only when there is sufficient
evidence to do so, and, where there is, whether it is in the public interest to do so.
You might think there is (in both tests). I might even think there is. But we don't decide, and I suspect the public authority would have enough of a defence to mean that a prosecution would not be appropriate.