The author's interpretation of the argument in favor of dynamic languages seems purposefully naive. I don't think that any proponent of dynamic languages or unit testing claimed that the mere presence of unit tests guaranteed bug free code or that it was impossible to have type related errors at run time if you have unit tests. It's a more nuanced argument asserting that the benefit to programmer productivity when using dynamic languages outweighs the cost of potential type related errors not possible in a statically typed language. Whether it has merit or not is the question I hoped this paper would answer.