It means that the car is too quick for its suspension on B roads. It’s not set up to be able to monster these kinds of roads and it shows a real lack of composure beyond a certain point, it’s a very sudden change and tells me the suspension folks at Tesla had a choice to make given how the car behaves at lower speeds I.e. excellent. There is always a trade-off.
My old Clio 172 cup on custom KW coilys would always reward harder driving on B roads, it just got better and better as it got pushed harder - but I was younger and more foolish then.
Oh, and I don’t watch Top Gear, not since the Tiff Needell days anyway.