Not sure whether this is proven but if it is true, then i understood this part of what he said
"So if you defend with 20 width divisions that means you begin trying to reinforce the battle at 75% strength rather than 50% if you had 40 width divisions. "
Because in the same time frame (1 hr), if the enemy attack is ferocious that 1 40 width division will be dislodged, then that same amount of attack might not be able to deorg 2 20 width divisions. At most only 1 20 width will lose organisation and retreat, hence the battle is at 75% strength and reinforcement will come...
If you want to read more on this I did some testing lately:
https://forum.paradoxplaza.com/forum/threads/testing-defense-with-infantry-against-tanks.1382215/
https://forum.paradoxplaza.com/forum/threads/best-defensive-template.1380809/
In short:
Defense increases with batallions added to a template. Organisation is an average value. A 2width template with only 1 inf batallion has 60 org. A 20w inf has 60 org. A 40w inf has 60org. (Buffed and debuffed by serveral other things)
If you fill the 80 combat with with 8 10 width division they can have 8*60 = 480 org, 2 40w only have 120 org. That means against the same attacks (after defense!) the 8 10w hold 4 times longer. But the 40w has a lot more defense, so less attacks get through.
There are a lot more aspects like equipment and manpower loss, deorging the attacker, ... In the threads linked above there are some very good answers that explain some of that.
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