Remember that stacks of 8 are in some sense optimal, more units stop adding a stack bonus whatever that means. So the Dwagon's probably CAN'T all engage effectively at once since there are more than 8 of them.
Parson does know more of the rules, and he CHOSE to engage the Dwagons in stacks of 5 + 3 Warlords, almost as if more would be an active disadvantage or at the least no advantage at all. Parson's force was working it's way down the line with the line headed more or less toward Gobwin Knob, so relaying in "fresh" Dwagons DIDN'T save the Dwagons much movement, all those withdraws and relaying back and forth cost movement too, why in the world would he bother if 19+3 is much stronger than 5+3? Just move in all 19+3 and wipe the entire stack without withdrawing at all. But in fact he chose the repeated withdraw option, almost as if the presence of an additional 14 dwagons WOULDN'T HELP NOTICABLY. Maybe, just maybe, when they TOLD us more than 8 doesn't add to the stack bonus and that therefor you see lots of stacks of 8 what they ment is that more than 8 is no real help, rather than that 25 is vastly stronger than 8 since everyone gets to attack at once anyway under many of the interpretations I'm seeing.
Similarly until Ansom decided to commit the warlords he was sending stacks of 8 against the Dwagons in waves. Again this doesn't make sense if larger groups are substantially more effective. I think attack strength is ENTIRELY determined by your best 8 units and that extra units help only on absorbing damage.
Jillian appearently expected to be able to get away with ditching two warlords, if adding two more warlords would make her stack much stronger this wouldn't be true, but if a stack of 9 strong units doesn't NEED 2 more units because it's already about as strong as it gets then this is easy to get away with.
And if 19 nearly dead dwagons + 3 warlords isn't much stronger than 5 nearly dead dwagons + 3 warlords then that Jillian can probably take the stack is exactly what you'd expect, and if this ISN'T true then almost NOTHING that we have seen about tactics makes any sense. They're FAR too willing to split up into groups of 6-8 for it to be true that a group of 22 is all that much stronger in actual combat.