A lot of people think of Impetus and Responsibility only with respect to safeties and touchbacks. That is a very common misunderstanding of Impetus and Responsibility. Most all of us, myself included, tend to use the words Impetus and Responsibility incorrectly. Most of the time, when talking about who put the ball into an end zone, we use Impetus, which is incorrect. The correct word for that is Responsibility. "Impetus" is just the physical action on the ball that causes it to travel in ANY direction. "Responsibility" is who imparted the impetus on the ball that caused it to be on/above/across a goal line when it is declared dead.
If you read Section 8-7, there is no reference anywhere in that section as to the result of such impetus and responsibility. You have to look at 8-2, 8-5, and 8-6 to get the result of responsibility for the ball being on/above/across a goal line when it becomes dead.
Touchdown
8-2 tells us that, when the attacking team has legal possession of the ball when it is declared dead on/above/across the opponent's goal line, that is a touchdown. That rule makes no reference as to which team is responsible for the ball being there, so the result is touchdown, regardless of who is responsible. But, make no mistake - one team or the other provided the Impetus, and is Responsible, for the ball being there. (Note: AR 8-5-1-XI does not seem to follow this rule, with no good, apparent, reason. That question has been posed to the 'power that be.' We'll wait to get an explanation from 'the power.')
Safety
8-5 tells us that, when the defending team is responsible for the ball being on/above/across their own goal line, in their team possession when it is declared dead, the result is safety.
Touchback
8-6 tells us that, when the attacking team is responsible for the ball being on/above/across their opponent's goal line, loose, or in the opponent's team possession when it is declared dead, the result is a touchback.
So, yes, Impetus and Responsibility apply to every incidence of a ball being dead on/above/across a goal line.