It is true that some things you just need to test, but nullability isn't actually one of them. Many languages—including SML, Haskell, Rust, and Swift—encode nullability in the type system, so those languages can guarantee at compile-time that functions never receive null values.
You read the comment backwards. It was saying exactly that nullability of the argument is something that can be guaranteed by a type system but you can't write a test that makes sure of it.
