We aren't talking about AI or how the data is collected. The point is that this data would have been collected by the first-movers the entire time they are on

We aren't talking about AI or how the data is collected. The point is that this data would have been collected by the first-movers the entire time they are on the road. Such data would not be made available to newcomers. If a new self driving car company would join the scene, they would need to gather all this data again themselves, somehow. This is the "uncopyable advantage" mentioned by OP.