Honestly, I think that quote is a small piece of tactical advice. It is saying that the strength of this army is the combination of elements, unlike most armies who have their obvious strengths and weaknesses.
If you really want an army wide special rule... maybe something like:
Faith, Steel, and Gunpowder:
As long as there is at least one Warrior Priest or Arch Lector, one unit of at least 10 Halberdiers, one unit of at least 10 Spearmen, and at least one war machine on the battlefield that is not fleeing the entire army gains +1 Combat Resolution and may reroll all fear, terror, and panic tests. These units/models must be on the field (and not fleeing), otherwise this rule is lost.
It may be similar to mark of slaanesh for chaos, but it would make sense that units would be less likely to flee in general if they knew they were being backed up by warriors priests, war machines, and of course, more state troops. I also felt that adding spearmen to the special rule gives more incentive to take them, sense spearmen and halberdiers are the fluffy
mainstay of any imperial army. Also, I think this would help solve the NEED for a BSB, especially in smaller games.