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.

Additionally I've noticed a loose correlation between people who lie and who their parents are or how they were raised. Nothing scientific of course, but people whose parents don't hold their feet to the flame are generally more likely to think that they can get away with something because "the other guy is stupid". People whose parents are either very intelligent or hold them on everything they say are generally more practiced at thinking of the various possibilities that can come about to refute something they would say.