Yeah, sure. But there's a difference between "buffs only active when in the main hand", "buffs only affect the main hand", and "buffs only active when in the main hand and only affect the main hand".
Clearly it works the same as additional effect: stun would on some Lv.72 Katana, it only applies to that weapon and to be specific only when in main hand similarly to how REMA apply their additional bonuses and access to unique weapon skills.