Michael Horn is woefully ignorant or lying.
A big company like VW, especially a German company creating products with people's lives at stake, must have a software development process. I'd bet its more sophisticated than say your average game house. Probably very waterfall oriented with sign-offs along the way. The software engineers must have been implementing and testing against requirements. And those requirements must have been signed off by management.
If that weren't true then VW should shutter its doors forever, because it would simply not be competent to produce people-movers with embedded software inside.