My interpretation of it was that the cd is removed for each enemy w/out pain after it's used, so I don't think it needs to be changed
It'd be kinda OP if Sion could wipe enemy buffs away every other turn if they were resisting pain for whatever reason.
Now if he had another unique that read like- Reduce Sion's cooldown by 1 for every enemy without pain
that would make no sense because it's not attached to a move and has no detail of when it would proc. It would need a phrase like 'after Sion's turn, reduce....'
and again, that'd be OP. Could end up with 35% prot regen every turn if you skipped the zeta