That's not correct. If I'm sitting in my car and it starts to roll downhill, it's gravity that's causing the movement, not my car. Yet applying the brakes is still the correct course of action.
Moreover, even if we assume that humans are the cause of the problem, it doesn't necessarily follow that the cessation of human activity is the optimal course of action. There may be other effective alternatives (e.g., large-scale engineered efforts to increase planetary albedo via cloud formation) that are significantly cheaper. It might be that the costs of mitigating the problem exceed the expected damage that the problem is expected to cause.
To be sure, I don't know that either of those possibilities hold in this case, but it's mistaken to simply assume them away.
If we aren't causing it (or contributing significantly), then stopping what we're doing won't fix it. So yes, it's important to the conversation that we're actually causing the problem.