Well, he probably believes the story himself. he probably got the koi from his father or grandfather and was told that it's really old. the average life span last century was only 55-60 years. so if his grandfather got this fish in his 50's, it is possible for the koi to outlive two generations of men without being over 100. the fish is probably 50-60 years old.
carp is the most commercially bred fish. shouldn't there be more reports of them living over 200 years?
also, notice how people who claim to be over 120 years old are always from some godforsaken place. there is a reason for that, they dont have passports over there or 2 or 3 different family members use the same passport. same deal with this koi