I am going to have to disagree on the conclusion being valid- more garbage in garbage out if they do not break out the style of pitching- would you say the same if it was a study on cars and they just included cars and did not tell you if there were electric and gas cars in the study?
Pitching injuries are repetitive stress injuries--that is, doing the same thing over and over and over again. Carpal tunnel syndrome is the most well known of these types of injuries.
Whether a pitcher uses IR or HE doesn't matter--it's the repetition of the motion that causes the damage. (IR is a more efficient way to pitch...but it still involves accelerating the hand to 60+ mile per hour over and over and over again.)
There is a risk to children from over-pitching. The fastpitch community at all levels--HS, TB, and college--prefer to ignore the risk.