[Kameron8]

As far as I can tell, the sub-attack skill always deals magic damage to the target, regardless of the weapon being used. To test, I equipped a Hikage, Tsukikage, Vorpal Fang, and Seiryuu Tessen in my off-hand, then Sub-Attacked a creature. Each time, it dealt magic damage, per the combat log. These weapons did not deal magic damage when they were moved to the main hand, or when they were used with an ability in the off-hand with Ambidexterity.

Critical Hit! Hirota Nobuhisa attacks Kinu with Seiryuu Tessen and hits them!
Kinu takes 214 Pierce magical damage. (Seiryuu Tessen)
Kinu takes 25 Lightning magical damage. (Seiryuu Tessen)
Kinu takes 34 Wind magical damage. (Seiryuu Tessen)
Hirota Nobuhisa recovered 2 FP.
Hirota Nobuhisa recovered 5 FP.

This is potentially related to this thread.