Lightning Exposure doesn’t require the main skill to shock.
Lightning Exposure makes spells hit for less damage BUT inflict Lightning Exposure on Enemies that are ALREADY shocked, doesn’t matter how, they just need to be shocked for Lightning Exposure to be triggered.

Following part is taken from the ingame “Shock” description”.

LIGHTNING DAMAGE FROM HITS CONTRIBUTES TO CHANCE TO SHOCK ENEMIES. THE HIGHER THE LIGHTNING DAMAGE DEALT, THE HIGHER THE CHANCE. BY DEFAULT A HIT HAS 1% CHANCE TO SHOCK FOR EVERY 4% OF THE TARGET'S AILMENT THRESHOLD DEALT.