Not quite EVZ. Flack guns were capable of independent fire, and IIRC, the guns usually just oppened up on targets the spotlights caught (its a known, pinpointed target, which offers better chances for a kill).
And while it was a barrage, they weren't just firing wildly into the sky, hoping the fragments will do some good, as you seem to suggest. If that were so, then firing at a fixed point somewhere within the bomberstream, and not adjusting fire, would be just as effective.