Ok as promised I have tested this skill and the multiple target increase works fine. Tested with Bullet Time, Conflagrated Barrage 2 and Buckshot 3, all of them hit upto 3 more targets whilst Flak Attack is active.

However, the extra damage component of the skill is missing so I will fix that in the next update which should be quite soon. I am also going to add some attack speed to the skill since for a Sniper, an extra few targets on a few AoE skills is kinda bad for a skill so high up in the 3rd job tree.