Both Pow and key have this uncanny power with Zir’an to survive whatever you throw at them, eke out advantages of an extra heal proc here and come back from disadvantages. They also both know how to play Zir’an in a range of styles, not just the monkey mode Flameblood Warlock deck that I like that was used as an example in the top post, I’ve seen things like Synja and Blue Conjurer from these guys, they care about the lategame and know how to get there and what to do once they’re there. Like Blob said, the longer the games goes the more mana and heal tiles you will have, this means more heal procs, which Zir’an thrives off, since it double dips offence and defence for her. There are definitely late game threats that Zir’an can struggle with (EMP into Zendo is pretty good) but arguing they don’t have viable lategame from an even board state rings untrue to me.
Obviously there are decks with a better endgame, but if the Zir’ans you’re playing against don’t ever have a good lategame at all then it’s probably because they tuned their decks toward the early game and are not playing the deck to it’s skill ceiling.