I half-agree and half-disagree with this.
I agree that trying to model the Primordial War with game mechanics is a bad idea.
But I don't think you can write off armies like this.
There are five castes, and only one of them is the "warrior" caste, and even then one of their legitimate focuses is War in the sense of, well, armies. Members of the other five castes need not be combatants. This being Exalted, many of them are, but one of the things I appreciate about Exalted is the way it legitimizes the idea of a Performance hero or even a Bureaucracy hero.
Sure, some of the noncombatant Primordial War Exalts were part of an Exalt-focused logistical train that focused on making sure each Dawn had a fancy daiklave - but I think it's depressing and a little too reductionist to see that as the only possible contribution. I think regular, human armies were valuable in the Primordial War. I think human societies were valuable in the Primordial War. If the Primordials are in part themes and ideas, then a Zenith who convinces humans to reject those ideas should be contributing to the fight directly. And so on.
I want the idea of a brilliant Solar general who led her army (including mortals) through a hidden pass to stage a surprise attack on a Primordial to exist, in Exalted.
To the degree that this is inconsistent with some elements some folks consider canon, I think those elements should be amended - because otherwise the UCS's division of castes doesn't make tons of sense, but that division of castes is what makes the game so appealing to me.
So there is two things here though.
First, the above contributions still require being made in the context of
an apocalyptic conflict of unprecedented scope, which means there is
no single mortal army
big enough to fight the Primordial host and win. Because any such army must encompass ALL of living humanity without exception, down to every man, woman and child. When you introduce mere mortal beings into the equation, like what happened with the Dragon Kings, the all-or-nothing stakes at work at such magnitude mean it becomes a war of fight-or-die imminent extinction for those below a given innate level of godliness.
Once a mortal force becomes a link in the chain of the Exalted war machine, it can assuredly assumed to be the weakest, and therefore a consistent priority target for everything which can be thrown at it. If the Primordials could not kill the Exalted directly, then they certainly could cut out everything underneath them and annihilate their spiritual/social infrastructure and future hosts en-masse with a blink of an eye. Because that is how Primordials
fight, not on the ground with armies and ideologies, but across whole species and concepts. And unlike naturally finite humanity, they will always have More to draw from. When a world rises up against a universe, the universe
has an edge.
Secondly, none of what you said actually contradicts any of what I laid out either. The context of a Dawn wielding a mighty army in total war towards a facet of existence does not preclude that army being largely as impactful and superficial to that conflict as the number of fingers on her right hand. It simply means when placed against the sheer enormity of the enemy she is facing, that she has adapted the circumstances of her focuses into a battlefield of her own designs. Instead of leaving it to the will of her foe to alter favorably or unfavorably as it wills, or to the neutral and inert environment which has no meaningful applications on such a scale.
You cannot abstract the Primordial war
up into a more meaningful conflict by attempting to contain it, you can only reduce it
down into being a border skirmish along given values of infinity.