Question about F-S Magnum rifle modding

Hello i found on Alpha miniguide this video -
F-S magnum rifle

Can someone explain to me why he has this weapon almost fully modded with toxic +% dmg fuels? I always thought these mods boosts only proper elemental weapon.

I am clearly missing something and dont know why. I will be glad for answers.

Mods can add damage of different types, not just the types on the item. It’s attack strength modifiers that will only apply if their respective damage types are present, which includes damage bonuses from mods.