But they really didn't want the overland speed to affect tactical speed.
I can understand that. They are not always related.
At the farthest end of complexity, it would be a separate stat. Since we need to combine stats to keep the game manageable, what makes the most sense from a design/logic/esthetic viewpoint?
A. Combat Movement is more closely related to overland movement.
B. Combat Movement is more closely related to how fast you can swing your weapon.
Or, another way to look at it:
Are combat movement and attack speed related. Should it be possible to have slow moving units that attack a lot, and fast moving units that attack slowly.
If we want the above combination, then the abilities need to be based on different stats.
Let's look at an example of giving your sovereign a horse.
Should it effect overland movement: yes
Should it effect combat movement: yes
Should it effect number of spells/attacks per round: no
Is this a strawman argument? I don't know. Probably. Can somebody think of any counter examples?
As a side note, the overland speed doesn't have to directly correspond to tactical movement. There could be a multiplier in there somewhere.