It's irrelevant. Emperors are invincible monsters, Luffy is suffering to "solo" one and Big Mom required Kid and Law fighting to death and exhausting their awakenings. "Zoro ap now is stronger", "Kaido durability is on another planet", "not taking many attacks" is classic hypothetical gibberish from useless power scaling: Oda isn't drawing Zoro soloing Big Mom now, Zoro doesn't belong there but with the top subordinates of the true monsters, that's his milestone and his present place in the progression of the story. The author doesn't intend the defeat of an emperor to be seen as such an easy task that one of Luffy's subordinates that was paired with a 1.3b lackey could do it now, it's just narrative, always have been. Big Mom's narrative as an adversary exceeds current Zoro's as a fighter.