Not necessarily. While Petrov may still have realized that the supposed incoming missile signals were a fluke, his higher-ups still might object to his actions. The only acceptable variation (that doesn't include nuclear war) would be that:
A: Petrov's superior also notices what's happening and orders him to fire back. He does not do this, to increasing angry demands; ultimately his office is stormed. Just before he is arrested it's understood this is a fluke of atmospheric reflection, and the button is not paused but only at the last few seconds.
B. Despite Petrov's thoughts, that this was not a real attack, he is nevertheless arrested for shaming the USSR and disappears into the gulags. His identity would be found out in the West, and, like Raoul Wallenberg, he would become something of a lost hero.