Its not in generally; statically-typed languages (e.g., C, C++, Java) make impossible certain classes of runtime errors that occur in dynamically-typed languages, but don't actually reduce the need to test each path through the code you've written to assure correct results, nor do they reduce the number of defined paths through the code assuming that the defined interface is adhered to (they make it impossible to not adhere to certain parts of the advertised interface, but even in a dynamic language you don't need to test undefined cases, as there is nothing to verify, so that doesn't actually narrow the test surface.)

Haskell's type system and syntax are richer in ways which I think makes it possible to unambiguously describe more results in code in ways which makes unit testing certain aspects superfluous. I'm skeptical of most "you don't have to test" claims, but I can see more of a case for reducing the need for testing with Haskell than with many other statically-typed languages.