Think about it. With all the backslash of the gaming communitiy that anonymous has experienced, wouldn't that just increase 10 fold if they admitted to being the ones doing it ?
If they really did it, the intelligent solution would be keeping quiet, that way, people will be more likely to blame sony as there isn't anything else to blame other than "hackers" in general.. Which is vague.
I don't know if they did it or didn't do it... But if they did, saying they did might not be the best of ideas.