It was potrayed as Luffy being immune to Blunt force back then. I suppose its something to consider that Luffy's certainly durable enough around this era to at least eat and endure Asgard Moria's punch which is beyond Oars and thus beyond Numbers too? I have to imagine that punch was a large share of the damage/pain Kuma pushed out of Luffy for Zoro to endure too.
That's not immunity (in the first place, its an NLF to claim he could endure beyond what he'd shown by feats, what he did with Sengoku being an outlier as we know with Haki he'd have ignored Luffy's devil fruit entirely), not to mention he still sustained the trauma from the beating he received, ergo Zoro had "nothing happen" because of Kuma.It was potrayed as Luffy being immune to Blunt force back then