Did Kaido and Big Mom attack at the same time? Yes, it was the combination attack that they use against five supernovas.
Did Zoro alone stop the attack from reaching the supernovas and save them? Yes, he temporarily blocked it with haki barrier.
It's not that hard to understand what occurred.
So now countering a devil fruit power with haki, an ability we seen before from "mere commander", is considered a 3 billion berry feat?
Chopper future opponent must be stronger than Kaido and Big Mom.
Let me know when he reach rooftop Zoro level.