Like a lot of things, Purify is an Ability as well as an effect.The same is true for Bleed: Nick Fury can use his Bleed ability on Colossus as much as he wants to: his Bleed ability works just fine.However the Bleed effect doesn't happen, because Colossus is immune to the Bleed effects. However, we can literally see that the Ability worked, because a little 'Immune' callout pops up.It's not immediately clear from his description, but Apocalypse works by reducing the Ability Accuracy of the Purify Ability. He targets the ability, not the effect.He can't reduce the Ability Accuracy of Cleanse, even though both Cleanse and Purify abilities eventually lead to the Purify effect.By contrast, Bishop inflicts a Stun passive if a Skill opponent Purifies a stun debuff, i.e. they successfully activate the Purify effect against a stun.Because Bishop targets the effect, he works regardless of whether the effect was generated by either ability; Purify or Cleanse.🙂