So much stupidity here. If Twitter are responsible for autoplaying then the sender is more responsible because they sent it.
But seriously can't you see that the sender knew that it could cause the receiver harm, their tweet stated that was their intent, it caused harm, they are therefore responsible for that outcome.
Imagine I flew a drone into someone and they were hurt by it. I knew flying the drone I to them would hurt them, but I didn't make the drone, I didn't make the power that enabled it to fly. I didn't make people fragile enough to be hurt by the drone. But I directed it with intent, just like the sender did in this situation.