I get that it could be a one-or-all kinda firing, but in my mind, a species that could build the Halo array wouldn't be limited by something as trivial as light-speed-lag; nevermind the fact that it was supposed to to be an "end-of-days" solution, implying that it needed to hit everything, everywhere, all at once.

So hitting the "fire all dem bitches" button should be instantaneous, to the point the flood couldn't survive by slipping in between the firing patterns...