From my standpoint, there is insufficient information to conclude what has happened. The police believe that no one was in the seat, but police also tend to be routine liars [1]. Without having the facts they used to draw that inference, I can't endorse their belief.

For my part, I'm also curious how easy it is to actually switch from the driver's seat to the backseat of this kind of car. I know in my personal car (a sedan), it is very difficult to move from the driver's seat to any other seat, which makes me discount scenarios that involve the driver quickly hopping in any other seat. But internal car layouts differ, and I don't know if this specific model is much easier to transfer.

Another thing that is woefully unclear to me is a) what Teslas actually do to make sure the driver is driving when using Autopilot/FSD/whatever, and b) what they actually do when they think the driver isn't paying attention. I've seen many contradictory reports on these topics, especially when some people say that Tesla says this is what they do yet videos seem to indicate that they aren't doing that.

[1] That said, this is also a case where I would have a priori expected the police to be lying to blame the driver instead of the car manufacturer itself.