Some people actually write tests.

We actually picked a fairly conservative number - there are even larger automotive codebases today.

For example, Mercedes’ MB.OS: “is powered by more than 650 million lines of code” - see: https://www.linkedin.com/pulse/behind-scenes-mbos-developmen...

650M LoC is certainly not a single codebase that you can "checkout" and "build". Also, the figure is a little bit hard to believe.