Likewise, lower intelligence might reduce personality differences and increased coordination. Individual troops with less foresight may be more careless of their own safety and thus more fierce on the battle field. In the manner of bees. Also, in a warrior society, obedience maybe favored over independent thinking.
Also, in an pre-agricultural societys everyone is a generalist: everyone must be smart enough to know how to do everything. Less true in agricultural societies.
So it's just as possible agriculture made people dumber but more dense population centers and thus more effective militarily.
My point is that barring actual measurement, we can conceive a great many possibilities and at this point no measurements indicate intelligence change over that period