It can be interpreted in two ways, The one most people take it as meaning Jiraya was in disbelief that any one man could defeat Hanzo alone, Though I personally don't believe that's what the scan meant...Considering that would mean that Jiraya is putting Hanzo above everyone he knows or knows of , including the likes of Minato and Madara..
2nd way to interpret is that he meant, he couldn't believe one man could accomplish the task of killing Hanzo alone... Note of the bolded, What I believe is meant by that is that Jiraya is in disbelief once person could defeat all of Hanzo's forces AND Hanzo himself alone... The guy he's getting the intelligence from even goes on to explain how heavily guarded Hanzo was
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..right before Jiraya makes his comment
In context, this is what Jiraya is most likely refering to.. that's it seems impossible one sole man could defeat all hanzo's guards then defeat hanzo himself all alone.. Such a task , to Jiraya would need a platoon of extremely skilled shinobi..
