For example, we might see ways of expressing indirection for testing that then get removed when building the final product. Or we can look at the great strides made in virtualization since the Linux kernel was started. Something previously hard to test becomes much easier. And I'd expect more so if the testing had co-evolved.

Of course, there still might be hard-to-test areas. But given the massive ingenuity applied to the kernel development process over the years, I expect that they would have advanced what's possible in TDD if they had pursued it.