True that, but according to the video, barracks were hit in several volleys.
Also the volleys were not simultaneous, but were timed out, with enough timing
to allow 'first responders' to be out looking for the wounded and assess the damage, when the next volley came in.
This seems to be a typical terrorist tactic to maximize casualties
That there were no fatalities was purely accidental.
I’ve been in enough “Bunker Dives” to tell you that no one leaves shelter just because there hasn’t been incoming for a few minutes. Once you are in there, you wait for the all clear to be given (or other instructions) before you exit.
With the surveillance systems that the US has, the command personnel have a real good idea if there will be any more incoming, whether it is rockets or mortars.