Disclaimer: I personally believe some weapon skills should be grouped together like they did with gun specialties in the pyramid article.
That said, Saber like the one used by the skill was used not in combat but in academic fencing so their usage differed from usual battlefield broadswords, favoring fencing just like rapiers but their reach and general form factor makes its handling differ from rapiers also. One can see where the authors saw a need to pull a new skill to handle this weapon, though I believe it's an unnecessary bloat.
Cavalry Sabers are used on horsebacks and you don't fencer on horsebacks, so you use broadsword skill.
Mechanically, it defaults to shortsword at -4 so you can get Weapon Adaptation (Cutlass to Saber) and set sail to rob merchant ships.
By the way, it's swing damage has nothing to do with it's form factor. Just look at how shamshirs uses thrusting broadsword stats. Sabers has less weight resulting in less momentum which means less swing (lever) damage. "Cuts" as you would see in fencing uses thrust damage, using the movement of wrists to slash instead of using the whole arm and the sword as a lever to increase momentum.