4 decoys hitpoints are more than the whole rest of my army (shooters, howitzers, apcs and tactical helicopter), and that triggers it just fine. Assuming those are atomic or global decoys, those 4 are over 160,000 HP - well over my entire normal army, not including generals and mercs. That is more HP than 5 heavy tanks, which is basically an entire army.
So the lack of firing means either:
This building ignores decoys
There is something beyond just raw hitpoints triggering them to fire. Maybe a combination of dps and HP and unit count?
I have also noticed that my decoys seem to be ignored, even when dropped in situations where they are definitely the largest cluster of HP on the board.
But these are great tests, it definitely appears that the silo behavior isn't exactly as they described.