How do unit tests prevent one from committing type errors in the "product" code? All it does is show you that you know how to write test code that doesn't supply the wrong inputs to functions.

I'm not shipping the tests. The fact that they work is very nice, and it can prove that the code under test is working, not that all attempts to use the code in something larger aren't broken.

So all that's left in this article is that dynamic languages make for easier to refactor code. That might be true, but it doesn't have to be so bad in practice when you keep your interfaces very small and focused.