I have a suspicion about this one as a major victory is very dependent on retreat logic. My guess is that this scenario wasn't regression tested.
In the final round of AGE1 and AGE2 updates, Pocus made quite a few changes for retreat behavior. In general, what he did was to make the behavior more logical/intuitive. (Also, to kill an endless retreat loop of bouncing back and forth.)
However, if you don't set a constant for military control to 5%. Then, it is really hard to destroy large armies as they slide away from you, and there is no way to box them in. It might take 10 turns of so to destroy Spartacus' Army, but you don't have those turns.
If you turn on the 5% option, it makes it possible to trap an army in a region. I have yet to do that. But with it on there are often multiple battles per turn and confrontations result in much higher losses.
Actually, I like what was done with military control (at least at 5% threshold). Otherwise, you would need to garrison every province or maintain a front line. But with military control + 5%, you get that behavior pretty much. Thus, freeing you from tedious micro-managing. You shouldn't get enemy retreats into your territory. And a Sherman's March is quite possible, but carries much risk too. Because if you should face an army stronger than you, you will be wiped out deep enemy territory.