Beware the consequences of "Armies are not Stacks".
But I feel I think I understand the significance of that statement.
I am not really sure about what kind of message Frogboy wanted to pass, but if it can help, here is a summary of my experience with Beta 1 combat which was only with hand to hand weapons.
The combat was between a single unit and an army. It was either my super combatant sovereign againts wandering armies or group of NPC, or a troll against my super combatant sovereign and some cannon fodder.
Each turn works the same way: the single unit attacks the easiest target in the opposing army while ALL units in the ennemy army attack the single unit. The combat speed of each unit determines how many attacks a given unit can make in a turn. A single unit will attack different targets in the same turn if it hasn't used all its attacks for the current turn and its first target is already dead.
It isn't a one to one combat. The important thing is that ALL units in an army are engaged in the combat.
I guess that Frogboy is refering to stacks as in Civilization's stack and not as in MoM's stacks or MOO's stacks (which are effectively armies as they all fights during the same tactical battle)
Battle rating can help to sense the odds of winning/loosing a battle. But it can be misleading as you can see with this post https://forums.elementalgame.com/380340
And it may difficult to have a correct sense of the outcome with only the battle rating since the result will depend a lot on how many damage each side can make on the other. Even with only attack, defense, HP and Combat speed it can be tricky in beta 1 to estimate if you have a chance to kill a troll (lots of HP, high attack, no defense, low combat speed). So it will be more difficult with range attack, type of damage (if different type of damage are available) or immunities (I am pretty sure that some mods will have some kind of weapon immunities for undead or werewolves)