Even if he had 30 round magazines, that’s 60 shots. Reports indicate more than 60 casualties and fatalities. Even if he didn’t miss with a single shot (likely?), he had to have changed mags.
I’m just speculating here, but to give the VTI people the benefit of the doubt, it is actually possible to reload one empty gun while holding another (ready) gun in your other hand. It is terribly slow, but it does have the advantage of being able to hold the unarmed at bay while you do it.