That document appears to be over 4 years old, predating the availability of Apple's Advanced Data Protection system that claims to provide proper E2EE on most iCloud back-ups. The latter was controversially the subject of a specific legal attack by the British government using the Investigatory Powers Act resulting in Apple withdrawing the feature entirely from the UK market rather than compromise the security of their system - according to public reports anyway. Before ADP much of the data stored in iCloud backups was not fully end-to-end encrypted and Apple itself did not claim otherwise.