You are correct. From
http://www.nfl.com/fans/rules/definitions:
"Impetus: The action of a player that gives momentum to the ball."
For the purposes of this rule, the action that gave momentum to the ball is the runner losing possession, not the defensive player knocking it loose. The motion of the defensive player would have occurred even if the Dallas man held on to the ball. It was his reaction, losing his grip on the ball, that caused the fumble. The rule was interpreted correctly.
For the purposes of this rule, the action that gave momentum to the ball is the runner losing possession, not the defensive player knocking it loose. It's interesting to me that you give a cite for the definition of impetus and then reject that definition. If the fumbler who had position places the ball down on the ground so it is motionless, the ball has zero momentum. A player who dives for the ball and casues it to squirt away, even if he is on the opposing team, has given the ball momentum. He has provided the impetus.
ML/NJ