The ground can't cause a fumble thing means if you ALREADY have possession, as in you have the ball in hand and are running down the field, and you (for example) get tripped up by a defender, stumble and fall where the ground dislodges the ball. THAT would just be down by contact. The issue is the Lions game was that he apparently hadn't shown control of the ball before letting it go. He wax still rolling from the initial catching action.