It's wishy washy. If the crew had been drinking and were subsequently breathalysed they could say "there is proof alcohol played a part in this accident"...because it can be quantifiably measured. Short of someone actually falling asleep, how do you measure fatigue and hence prove it played a part?
Don't be too rough on the ATSB, as others have said, it's not that fatigue wasn't an issue, it's just that they cant 'prove' it..