I like weird questions like this.
I think a battery would have to be of several thousand volts across the typical 9-volt style terminals to have any chance of killing someone. I suspect there is about a 4 to 5 kV maximum before arcing occurs and so the battery would be 'impossible' to make.
It does sound reasonable to me though, 15 people per year. What that number probably reflects is people who had prior medical conditions.. heart attacks in response to tongue zapping (I gave this up long ago;) and so forth.
Popping ballons in grandpas ear probably does in more than that, for example.
However, to answer your question, I am convinced there is zero chance of dying from a 9 volt zap to the tongue. Current has to flow through the heart or the brain... ahh, and if the case is attached to the negative terminal, as it sometimes is, and the case is held in a wet hand, .. I see, the current flows through the arm via arteries pasing through the heart on the way to the tongue, yeah .. it could happen.. i changed my mind, but the answer is yes, albeit unlikely.