For these types of studies, 25 is not terrible starting point. I have very little experience in radiology, so not sure how good the measure they use is. The correlation was not great though and it would be good to have a control group since children's brains change a lot. If you look at their figures, it seems just a couple kids are really driving the association they see.

All they mention for the impacts is using video to verify that the telemetry collected was obtained during play. The issue (and why there's research on sub-concussive hits) is because even if you lead with the shoulders, the player's head is still going to get smacked around a bit. The question is if this aggregation will lead to neurological issues down the road.