I'm assuming it is Hashirama's ability to heal wounds without handseals on a level greater than Tsunade's as Madara put it. I'm taking this to be the case considering Sasuke got stabbed later and there's no need to deny that he would need a plot armour to survive. If this wasn't the point and Sasuke's going to die, then what was the point of Hashi giving Sasuke a jutsu in the first place? I don't think Madara was tricked with a clone or genjutsu, he's freaking Madara whose plot shield is greater than SSJ God Goku. Otherwise, Kishi would have to pull some sort of otherworldly asspull. Like rinnegan or some other shit.