If he denied they were, he would be asked further questions and possibly caught in a lie. Such as "is this the only time that your account has been used by someone other than yourself?" or "So you are saying that this is the only tweet that you didn't make but the others before and after you did". And so on. My guess is that he would be advised to tell the truth to prevent getting further trapped as far as his credibility. There may also be other people that he discussed the tweets with that could be brought into the picture as well under oath in court I'm guessing. Bottom line: Denial is easier said than done.