Blaming the wrong tool
Most Pokemon gyms & stops are at places where youths would normally congregate at anyway, so a fair number of them could have been killed while obsessing over something other than Pokemon Go. Like social media apps or sexting on their mobiles while arranging to meet their family, friends, boyfriend, girlfriend at the aforementioned places.
There's no real evidence other than just a random co-incidence unless the researcher has hard and fast evidence that the last app open on the unfortunate's mobile was indeed Pokemon Go. Which I very seriously doubt.