Not tightly coupled in obvious ways, but as I understand it they aren’t putting it on pickup trucks, convertibles, or anything toeing a boat etc. Their vehicles don’t have aftermarket suspension systems dramatically changing handling characteristics, or turned one into a stretched limo etc.

Which means the software can safely assume the vehicle will behave within a relatively narrow operating range.

I suppose owners will be motivated to have the thing do the driving (and so seek defeat devices and such), but at least the software can have "do nothing" as a safety mode if it manages to detect that the vehicle is not configured as expected.

And maybe the software can be designed to be coupled to a vehicle dynamics model that can be updated.

I don't think the vehicle performance really matters in the typical case. They're using like 20% of what the vehicle "can" do. They're probably hedging against the long tail of variance on the road somehow. Kinda like how private people can tow whatever the f they want with their pickups but in a work setting you need to keep it fairly stupid proof.

The only thing an autonomous system should do with janky modified cars is drive them very slowly to the state police barracks for destruction.