I'm not contesting this is a problem, and that he should not be corrected. Why it was done like that, first place? Because this helped speed up the pathfinding algorithm (and so the turn resolution) by a significant factor.
The two others parameters to consider in the equation are:
a) the AI won't be able to tweak dynamically its divisions to speed them up when needed. This means the AI divisions will always move at the speed of an artillery battery.
b) the terrain matrix is very hard against wheeled units in mud. Having a division with a single battery nearly halted in muddy weather don't seem very realistic either.
When the simplification was decided, I admit I thought of the argument of having units helping each others. Horses and mules in excess (or even soldiers helping the batteries servants) could mitigate in my opinion the loss of speed of the whole division, just because you have 6 guns to move with your 5000 men...
Also you have to figure that when a big body of troops move, they don't move always at the same speed, yet they still appears as a single counter. Over a march of several days, some regiments can be ahead, and the artillery can follow some kilometers behind, because of weather perhaps... but if a pause is decided, or if a battle ensue, the artillery train can keep up in a matter of hours, even if it implies continuing the move by night while the first line is already resting or is deploying for battle. In the end, when the sun rise, everybody is still in place.
So I don't think it is as black & white as you said, and I'm still unsure that a whole division should move exactly at the speed of its slowest element. I think some compensation should be in order, if only for gameplay purpose and leniency toward the AI.
That said, we will do something
