They didn't. They gathered tenuous research form other countries.
If you actually read their report it is all 'low confidence'.
Just as recently they said they were not budging on exams, because other 'very successful' countries were still running exams. Ignoring that those countries have fully tackled covid in a range of different ways so don't have the attendance issues we have. One of the countries named doesn't have terminal exams!!
Even Sweden is closing its secondary schools before Christmas!!