For what it is worth, this was down to a disagreement between the ICO and the civil service. It affected many government departments and goes back to the last government.
The civil service was of the view that emails to and from private accounts belonging to ministers and their advisers were not subject to FoI requests and that there was therefore no requirement to keep such emails. According to the advice ministers and advisers could delete such emails without keeping any records in the same way that most of us delete emails after we have dealt with them. This was the guidance the civil service gave to ministers and advisers under both this government and the last government.
The ICO was of the view that any such emails are subject to FoI requests if they deal with departmental business, although emails discussing party policy are exempt. The ICO chose to use the DfE as a test case and won.
Personally I am with the ICO one, although the dividing line between departmental business and party policy can be very difficult to determine. But I think any criticism should be directed primarily at the civil service for the incorrect advice it was giving ministers and their advisers.