I don't think that is true. Any one other than a highly skilled person who is able to actually shoot another human being would have done more harm than good. Research has shown that just because you are good at the range, you would be good in a real life situation. The guy in the theatre was a straight up psychopath, and the lack of empathy and emotion he would have had would have made him a much better shot than like 99.9% of the population.
For the average human being there would be two major problems when trying to shoot back at the killer: Firstly, shooting another human being is hard. Even if your life depends on it, for the average man taking a human life is difficult. Secondly, the theatre was dark and the loud screams and gunshots would have been disorientating. Add to that the extreme emotional stress, people running around, not to mention tear gas, it would have been less than optimal. On top of that the killer was wearing a bulletproof vest and a helmet, so a concealed 9mm might not have done much anyway, even if he did hit.
On top of all that, even if there was someone who could ignore the stress, the gas, the darkness, was quick thinking enough to pull out a gun and return fire, managed not to hit any fleeing civilians and then take the killer out of action, the killer would have still had enough time to kill and injure several people.
In reality what would have happened if someone returned fire is more likely that he would have drawn the attention of James, and if he fired possibly hit a couple more innocents, and then been killed.