Do you have a cite for this, or is it just wishful thinking on your part? There would be no reason to consider "impetus" if it always meant possession.
ML/NJ
You have to consider impetus because of punts in that rule which you sited. In the case of a punt the team is deliberately giving up possession, so from the point of the kick to the point of the catch possession is either neutral or belongs to the receiving team (largely depending on where in the field the ball is). Which makes who possesses a blocked punt somewhat confusing, thus they have to go with a different aspect of possession, that aspect being the attempt to move the ball forward, ie impetus. The punting team was the last team to attempt to move the ball forward, same thing holds with this fumble that bounced back, you can't be considered to be attempting to move the ball forward if you don't have possession (defenses are attempting to thwart forward movement, they don't own the ball so they can't move the ball), no possession no impetus. Seattle never had possession, so Seattle never applied impetus.
One must always keep in mind the disclaimer that appears at the beginning of any sport's rulebook about the use of the word "illegal". They're always careful to point out that "illegal" is an in-game term, not an indication of actually breaking some law in an actionable way. That holds for all words in a rulebook, the meaning in the outside world and the meaning in the book are often different. Impetus is another one of those words, in the NFL there is a strong artificial construct around the concept of impetus, it's not just a matter of Newtonian physics, it's about whose actions (the possessing team) are considered to be legitimate attempts to move the ball towards a defined goal (the opposite goal line).