The document you refer to speaks of the use of different tactics on the battlefield depending on the difficulty, the combat strength of the a.i. units affected by the difficulty level and of a morale bonus the AI gets on expert. I believe taken together these features can be equalled to a more sophisticated way the AI behaves being harder to overcome.
But if we forget the data provided by the Guide (for a minute): I had drawn the same conclusions experientially (by playing on different difficulties) long before I came across this information in the Guide.
As for your invitation for others to express their opinion, it seems that people at the Org have already discussed it some time ago and do not wish to repeat themselves. We are too late to have opened discussion on that topic, Victor.
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