I am quite familiar with that incident. It was an airshow at an uncontrolled airport. They had a line boss with a radio that had announced that the model aircraft were going to perform as scheduled. He alerted all the full scale aircraft in the area that the runway was closed to full scale traffic and to remain in the pattern until further notice. The Pitts pilot announced to the line boss he was going to make a low pass. He was told to abort multiple times but ignored the warnings. The model airplane was in a hover about 20' up when the impact happened. It was determined that the full scale pilot was at fault. In hindsight, the model pilot should have dumped his airplane into the ground when he heard the Pitts down low and close, however in the heat of the moment it's not as easy as we would hope. There would have been the risk of debris resulting from the intentional crash of the model.

Like you said, nothing is ever going to be 100%. If that were the goal for all of aviation then we simply wouldn't have any aircraft at all.