Doesn't it come down to intent, Cotch's hands are clearly on the ball before contact with Shiels is made. Therefore he was competing for the ball which is the cornerstone of the game we love, AFL. His intent was to win the ball and in doing so protect his own body from the impeding collision which is instinctive.
If competing for the ball resulting in injury to an opposition player is a reportable offence then there will be literally hundreds of similar cases every season. If a ruckman lifts his knee in a ruck contest and the opposition ruckmen gets a broken rib is that to be deemed intent, it opens Pandora's box.