It's been reported that he pulled down the signs, then pulled the fire alarm, and as soon as the fire alarm sounded, he ran away and exited the building through another exit.
That's not the appropriate response to an honest mistake. The appropriate response would be to immediately notify people that there is no fire or emergency, in order to prevent unnecessary panic and wasting of EMS resources.
Even if it was really an accident because of extremely low IQ, he should be disciplined for basically acting like a criminal.
More likely, everything he says is a lie, and he did it intentionally. When we see the video, it will be telling.