Take the same car to a modern city - and while you can still cause damage with it, it wouldn’t be as devastating since they know how to deal with cars and have the infrastructures and rules to safely deal with them. Bring a tank, however, and it’d be a different story.
Just because a tank is a more powerful weapon than a car doesn’t invalidates a car as a weapon. You can take a brick and go on a smashing spree in a populated city, and they will stop you fairly quickly, take a machine gun and you will be able to hurt a lot more people with it. That doesn’t mean the brick isn’t a weapon when someone uses it to kill people, it’s just a different level of weapon.
And yes, a K3 civilization will not consider a 10^15 watt ship trying to attack it as an existential threat like a sub K1 civilisation will, but a modern military won’t find a guy with bow and arrow as a threat (unless he is Rambo), still, a bow is a weapon regardless. It won’t win a war, but it can still kill.