He didn't underestimate Zoro's asura or his attack. Zoro was just that fast and powerful.
In the top right scene we see a big black "dot" and 2 small black dots together in the distance. That is Kaido, Zoro and Law. That gives us an idea of the distance between Kaido and them.
The next page shows the clash between asura Zoro and hybrid Kaido.
- Zoro enters asura mode.
- Zoro moves and appears in front of Kaido in an instant which surprises Kaido. The middle scene with Kaido's face shows a little white mark which tells us he was surprised.
- Zoro and Kaido clashes. Kaido tries to hit Zoro with name attacks but it's unable to because Zoro blocks or deflects all of Kaido's attacks.
- Kaido is then seen holding his club up so he can perform another attack.
- Zoro is too fast for Kaido and cuts Kaido first.
This scene just shows the distance Zoro traveled after he cut Kaido which is used to again display Zoro's incredible speed.
Compare that to Luffy red attack where he was aiming for Kaido's head and all Kaido had to do to evade it was move his head out of the way and maybe part of his upper.
Unfortunately, that also left Luffy open for an attack.
From breaking the 2 encounters down you can see that it was much easier to evade and counter Luffy's attack then Zoro's attack. Zoro also showed greater speed.