The author claims that the number of tests required are only those tests that are necessary to describe the correct behavior of the system and that this is true

The author claims that the number of tests required are only those tests that are necessary to describe the correct behavior of the system and that this is true whatever the programming language is dynamic or statically typed. But with these assertions you highlighted What about invalid arguments? What if someone calls: (random-elements -23 nil)?, the author shows that he don't give any value to static typing at all. He simply believes that what is tested with types don't matter: So I’m not going to worry about that. It’s on you.. I think he misses the fact that types are a cheap syntactic way to bring confidence on very diverse aspects of a system: compatibility of values or binary representations but also correctness of dynamic behaviors (see how Rust controls memory with scope type variables) or even algorithmic complexity (see A typed, algebraic approach to parsing https://www.cl.cam.ac.uk/~nk480/parsing.pdf https://www.cl.cam.ac.uk/~nk480/parsing.pdf).