If Jhin uses W on a champion that attack's on-hit spell effects will be calculated as AoE damage instead of single target. For example, when taking the Deathfire Touch keystone on Jhin, his W applies the effect for 2 (for AoE abilities) seconds on enemy champions and not 4 (for single target abilities). Deathfire Touch cannot be applied to minions, so the AoE aspect of W is irrelevant. I believe that his W is much like Yasuo's ult, where the main target is single target damage and the rest is considered AoE. This is because Jhin's higher damage ratio and stun can only be applied to the first enemy champion, and lower damage ratio is applied to minions.
1. Choose Jhin
2. Take Deathfire Touch keystone mastery
3. Level up W
4. Cast W on an enemy champion
Notice how his W applies Deathfire touch for only 2 seconds.
To clarify I am saying that Jhin's W hit on minions should stay AoE and apply spell effect to minions as such, but that the single target hit on an enemy champion should also apply single target spell effects.