You’re right. But the thing that’s interesting about the double slit experiment though is that it works on only a single photon. It’s as if all the traffic was created by a single car. So classically you might not think that the single car should care if the freight truck is heading down a different lane than the car but I’m QM it does, because the car is in a superposition of occupying several lanes.

I’m probably driving the analogy straight into the ground of course