Zoro stopped the attack for several seconds. Here's the sequence of events:
Mama and Kaido launch the attack:
Zoro then blocks the attack, and we see a massive shockwave that dwarfs Onigashima:
The shockwave generated by Zoro's block had a radius in the tens of kilometres.
The Yonkou are shocked that someone could stop their combination technique:
Zoro holds back the attack for a good while having a full conversation with the rest of the Supernova:
(While all of this is happening, Zoro is coughing up blood, showing that he's feeling the full impact of the attack).
We see the Supernova get out of the way, meanwhile, Zoro is still blocking:
The attack overpowers Zoro's block and explodes:
Zoro blocked the attack for long enough that:
- We see a massive shockwave
- He has a stare down with the Yonkou
- They react in shock that he can stop the attack
- He has a conversation with the other Supernova:
- He tells them to get out of the way.
- Luffy cries out in concern for him
- He warns them that they will all be annihilated
- The Supernova escape
- Only then does it overpower Zoro's block
The above sequence of events took several seconds.
- Zoro is not on the verge of death.
- Zoro withstood Hakai for several seconds.
Luffy, Kidd, Law and Killer would all have been annihilated had Zoro not stopped the attack. Zoro outright tells them as such:
If Zoro was not on the Rooftop, those 4 would have died in this chapter.
Blocking Hakai as Zoro did is a high top tier feat.