To be fair, government confirmation of a 'contact' 'inside the Chinese parties involved' seems like at least somewhat of a new development. Maybe not 'proof', but definitely stronger evidence - maybe even a step beyond suspicion.
The cables simply do not "confirm" China's involvement in the attack. Such confirmation, it seems to me, would be a blockbuster story worthy of the front page of CNN.
Have you found the relevant cables somewhere online? I've been looking and am unable to locate the actual cables involved in this particular story.
if the sources were deemed extremely reliable one would imagine that it would be enough confirmation.