This flashback was not good to kuma in terms of powerscalling
Kuma spent his childhood as a slave, then a few years as a pastor, then as a pirate, and then as a government slave again.
Kuma was taking minor pain from old people when he was Zoro's age and was getting rocked by it, unable to stand up.
Zoro spent his ENTIRE LIFE training his body. Like, ever since his introduction, training is constantly one of the most important things to him.
Kuma's life was spent trying to save people, using his Fruit to repel damage, heal people, and teleport to avoid attacks.
Zoro literally slashed into the wound Buggy made while fighthing Cabaji to show Cabaji trying to take advantage of his weakness was pointless.
When have we seen Kuma punch people in the face and do damage? Kuma is strong, but he hasn't really spent much time training from what we've seen. It makes total sense that Zoro, who is used to pushing his body to its ultimate limits, has more base endurance than Kuma.
Zoro clearly has some weird buddhist shit going on with Death, too, as shown by the grim reaper in Wano. Sometimes I wonder if the dude is actually cursed.
But, yeah, this chapter pretty much proves that Kuma put his stats into Devil Fruit power, rather than overall endurance. If he had trained like Zoro did, he would have been able to tank Luffy's pain bubble.