Also in the article, it gives two examples of people saving others, the security guard and the minimarket employee. This suggests, not surprisingly, that if you are more familiar with the surroundings and are there doing a job, you are more likely to be able to work out what to do.
I find the concentration on terrorist attacks in the article a bit odd though. There's a big difference between being in a bomb attack like 7/7 which is in terms of the incident closer to being in a crash or a fire rather than being shot at, and shootings are hardly a unique preserve of terrorists.