Ehh wtf, I don't really believe that for a minute.. When I was in high school there used to be rumors about a girl who masturbated with hot dogs and they broke off inside her, it's totally the same thing but different culture.. and who masturbates with a fish sausage, that's so unhygienic.. Japanese girls seem pretty prissy and I don't really think they'd be stuffing fish sausages up there.
I'm by no means convinced they're true, but that's seems like it happens more than you'd think. Think of how many people end up going to the ER for foreign objects in their ass every year, how many dumb things teenagers do, and how strong their libido can be. And though adults are known as prissy, the dirty tomboy teenager is also an incredibly common trope.