If the firer is not selecting a target then what is he shooting at? The invisible titan infront of that guard army?
He's not firing "at" anything. Pick a direction, pull the trigger.
The issue that is being debated is where does it say it isn't limited by terrain. Just because one part of the rules doesn't apply doesn't follow that none of the rules apply. Saying it is so and proving it is so using the rules as written are two different things.
Terrain is only invoked as a subpart of the
LoS check inherent to normal shooting (i.e., can you see your target). If you don't invoke
LoS, you never need to worry about terrain's effects on
LoS.
P1. Normal shooting rules require that you choose a target.
P2. In picking a target, you must abide by
LoS restrictions.
P3.
LoS restrictions include determining if terrain blocks
LoS.
C1. If you pick a target to shoot at, you determine whether terrain blocks
LoS.
P4. When firing a vibrocannon, choose a direction, and measure a line out 36" from the model. Anything lying on that line is hit.
C2. Vibrocannons do not pick a target when firing.
C3. Vibrocannons do not determine whether terrain blocks
LoS.