The point isn't that the software will have no bugs.
The point is that the basic car will have much less software and much more basic software and that it is much easier to test and ensure this software is in working condition.
Also, it is absolutely not true that the basic software has to be updateable. With care, basic functionality can absolutely be completely finished.
Imagine you have been in the market with ABS controller for a decade or more. It has really simple functionality, it should be possible to just bake it and never touch it again. Or maybe a controller that takes care of basic battery protection. Or motor ESC. When the components are simple enough and well defined enough, it should be possible to "finish" the software to a state where it never again needs to be updated. Or updated only in exceptional circumstances.