This explains a lot about DHH's code

Even if you’re talking about unit testing, the only way I’ve found tests to help design is by ensuring that I don’t regress when refactoring. That’s a very valuable attribute, probably the key attribute of testing for me, but I file that under “don’t create bugs”, not under “helped me design”.

From the comments by DHH on his own blog post.

This tells us that DHH  fails to grasp how the value created by regression testing using unit tests is different from the value created by designing your interfaces using TDD.