Can be tested with two examples
First example, non-primary weapon kill:
# Place down a vehicle with playable unit inside in the editor
# Place down an enemy unit equipped with a sufficiently powerful AT launcher some distance from the vehicle, and give it a 'destroy' waypoint on the vehicle
# When the enemy unit kills the player with the launcher, the death screen will indicate the unit's primary weapon, and not the launcher.
Second example, vehicle weapon kill
# Place down a playable unit in the editor
# Place down an armed enemy vehicle and give it a destroy waypoint on the player
# When the vehicle kills the player, the death screen will indicate the gunner's primary weapon, and not the turret weapon or the vehicle they used to kill the player.