The science is pretty clear, balls do leave metal bats with more velocity than when hit with a wooden bat. Not sure what study you are quoting other than perhaps they found no conclusive evidence that this speed results in more injuries.. but there is no doubt or debate that balls do come off metal bats with higher velocities.
Perhaps they were noting that the total number of injuries remained the same because the increased number of ball-velocity injuries generally matched the number of injuries which were no longer occurring from shattered bats.