Per the CBS article:
"This party also used sophisticated methods to remove all possible indications of unauthorized activity, and alter system times to cause further confusion."
So if the 'hacker' was so sophisticated as to remove all possible indications of unauthorized activity then how can they possibly conclude there was ANY 'unauthorized' activity? They would have to find some evidence of it (and they say the did not) to state otherwise.
So I can only conclude the 'forensic analysis team' was spouting pure unadulterated speculation with no basis in fact. Their analysis should really be 'we found nothing to substantiate any hacking occurred' rather than 'the hacking was so sophisticated it left no traces that the system was hacked.